Suppose you have a game where:
* You have an HP stat, which represents health, and which has a current and a maximum stat. The current value can't ordinarily exceed the maximum, and if the current stat reaches 0, you die.
* You can level up, and when you do, your maximum HP increases.

So, the question is, when you level up, what should happen to your *current* HP?

For example, if you have 12/20 HP, and your max HP increases to 30 after level up, what should happen to your current HP?

I've seen games that rise the HP a bit*, others that keeps the same HP and games that refills your HP at max.

(*) that bit based on a formula

You should continue to play the game if it entertains you.

Really, this kind of question would be better off on a gamedev discord, slack or even the itch.io forums.

In games where healing resources are already fairly plentiful and it's not too much about resource management: Just refill HP to new max.

In most games where healing is pretty limited and there's a goodly bit of resource management: gain to current HP the amount added to max.

In games where healing is quite rare, expensive, or a core conceit is resource management: no change to current HP.

It's better to err away from auto-heals if unsure. If you get a refresh of HP in the 3rd category, that can least to feels-bad situations when you level up ("I wasted a heal; if only I hadn't leveled up yet'), which should never happen. Note that some games could use healing on level up explicitly as their primary vehicle for healing in long dungeon dives, in which case it should be preserved. But in these cases, the game is usually carefully designed and paced around that (and usually have leveling systems that are more about control of pace than "grinding XP").

Yeah, I'm from the "Heal to Max" school of thought. Secret of Mana spoiled me.

The only exception are rare games where classes have growth exceptions. Like robos in SaGa 3.

You should probably mention *which* version of SaGa 3 you're talking about, because the two versions are radically different, and in particular they differ drastically in this respect.

In the Game Boy version, robot HP increases when you use a certain buyable consumable item, and both current and maximum increase by the same amount. (This behavior is actually useful for determining when you reach the maximum, as current HP won't increase if maximum doesn't, and the maximum is higher than the display maximum.)

In the DS version, robots get HP from their equipment, and I believe only the maximum is affected; the current score remains the same unless it would be higher than the new maximum.

Incidentally, it is the first Mana game, Final Fantasy Adventure, that soured me away from the full heal at level up mechanic; one time I played a (mostly) Wisdom only build, using spells as my primary form of offense, and I found that I never needed Ethers because I'd level up before running out of MP, which feels like it defeats the point of Ethers. I actually prefer the approach that Morrowind uses, where current HP increases by the same proportion as Max HP, so 12/20 becomes 18/30, as that's certainly more sensible than the full heal at level up mechanic.

The "fairest" is probably raising your health as many points as the maximum HP increase. But getting a replenished HP bar on level up feels very satisfying (and can literally save your life). :D

Have you ever timed a level up to make use of the regenerating/healing? I'm not that crazy about minmaxing but it could be possible if you're desperate not to use healing items.

It should drop down to 1 (one).

Have to agree with Sachys here. I just go with how the particular game in question works and adapt to it, without thinking it *should* be this or that. Would be pretty boring if all games adhered to the same rules.

Depends on how levelling up works:

A) If You level up automatically and possibly while in combat You should get the new HP - You'd have 22/30 HP now.

B) If You have distinct combat and levelling, so You'd only level after combat is concluded, You should get fully healed.

Generally it's a pleasant experience to be saved by levelling - no point denying that fun. The generous +50% HP example is level 2. No other level should offer that. Later on it'll be +5% at best. If You'd modify the gain by current HP % (You'd have 18 HP now) it would hardly be noticable on higher levels. If the game really does build upon antagonizing the player You'd rather go with Themken and set HP to one, else pick A or B.
While a boost from levelling feels good even a good thing can be overdone. The benefit should still not break the flow. To fully heal can turn a situation on it's head. Hence it should not happen in combat, so as to preserve combat drama.
If levelling is presented as a story in itself and "opens a new chapter", distinct from combat, a full heal feels appropriate however. Such level ups are often presented in full screen, bring their own theme and do their best to have the player focus on nothing but levelling up for the time being. The story before and after are divided due to it already, so a full heal is a warm welcome to a new chapter and can't mess with flow.

Usually, I see the beginning of a new chapter not associated with leveling, but rather associated with things like the following:
* Completing a dungeon, or defeating an isolated boss (an isolated boss is a boss that isn't attached to a dungeon); similarly, in stage-based games, completing the stage (Castlevania 1 is the sort of example I'm thinking of for a stage-based game)
* For story-oriented games, a major storyline event occurring
* Similarly, when the game changes its point of view to another party entirely (Dragon Quest 4 is a good example here, Final Fantasy 6 does this at one point, but FF6's multi-party battles and dungeons don't count here)
* For quest-oriented games, completing a quest
* Perhaps, in games with expensive class changes, a class change. (So, for example, Wizardry series, Dragon Quest 3, ant AD&D based game that allows dual classing, but not Final Fantasy 3/5 or Dragon Quest 6/7.) That might be the only one related to character growth that I'd really count.
* Also, I think being able to get a full restore in town or another safe place makes sense. (I actually don't like how many WRPGs subvert this by letting you rest anywhere except in town; why have that restriction?)

In any case, I don't see leveling up to be the start of a new chapter, but just a minor evolution in the character's abilities.

(Also, note that there are games that don't use levels and instead have more granular character growth, like most SaGa games. Then again, many SaGa games give you full HP (not LP) for every battle.)

That can still break resource management in longer dungeons, where part of the challenge is managing your resources so you don't burn out before the end of the dungeon, or you at least have enough resources to return to town when you start running low.

(Sidenote: I don't like it when games take away the option of returning to town.)