I've been playing quite a bit and have discovered a nasty bug that no one else seems to have reported yet. Any mission that provides the AI with reinforcements will start those reinforcements on the very first turn. This means you will be fighting two lances of bad guys at the outset which pretty much limits any type of tactics. On top of that, if you wipe those reinforcements out and then complete the main objective it will announce the arrival of those reinforcements then instantly completely the mission. Only once have I gotten a mission where I completed my main objective then had the reinforcements announced and show up.

You probably want to be over here:

https://community.battletechgame.com/forums

Here's how I found that:

https://www.google.com/search?q=battletech+report+bug

The forum you linked has officially been abandoned by the devs after the beta ended. The "active" one is the one hosted by Paradox here:

https://forum.paradoxplaza.com/forum/index.php?forums/battletech.994/

Yeah this has been troublesome in lower threat missions. In the mid threat mission (2 1/2 skull) that I am doing now it is absolutely brutal because it was supposed to be a mission to fight a single heavy lance. I have a medium cavalry lance for my best mechs and decided to risk it for the salvage. As soon as we entered combat however we were fighting two heavy lances, not one. That is quite a bit more threat than 2 1/2 skulls I think when they all arrive at once.

From what I have seen, the reinforcements appear as soon as you approach their spawn point. So if you run towards it (it is likely close to where the first lances starts), then yes, it will appear in the the first turn. But if you are careful, you might be able to get the first lance to follow you somewhere else (likely your starting point) without the second one showing up until you want to.

oh so it was a bug ?

it was the reason of the first battle i lost so far in my campaign (well, then, later on, i got brutally murdered in the SMithon mainStory mission)

at first i thought that, despite i accepted a 2/5 difficulty mision, i was in a system that was indicating 'system challenge 2.5/5' (or 3, dunt remembe... the galaxy corner where game kick you in after you recovered the Argo)

so i arrive and yes, the reinforcements were already triggered and active as soon as i spotted main lance contact

which means that while next main story mission was 2 skulls, and Lady Kamea asked me to "reinforce/improve my company's mechs" before coming mack, i was with 4 medium Mechs of 45-50 tons max...

and i got the following on my skin on very first round:

Heavy mech: Orion, and another Thunderbolt
Medium Mechs: Griffin (1x PPC), Wolverine, Trebuchet (2x LRM15), Vindicator (oooh, another PPC !)
Light Mech: Jenner, Panther (... come on.... PPC again ?)
+ at the far other reach of the map, 2 heavy vehicles
1 tank carrier with 3 PPC (fired it in short burst from god knows where on the map) and a 5x LRM20 carrier near it

I tried the mission thrice, trying to go around the first lanc to be as far as possible from the reinforcement and additional vehicules startint spots... didnt help at all...

Ok... it is intended, dont call it reinforcement, just tell the player "hello, you are flanked/surprised by two lances at once, good luck, hold still, have fun"

If it's a bug, yeah, correcting it could really help. I mean, i'm not a complete beginner in tactical games in general, and im not knew to battle tech either (having played the boardgame + rpg for 10 years) But here it's a bit punishing

Thanks for the link, I'll post over there as well, and yes, this is a bug, as I said, you kill off the lances then go to the objective and seize it only to have your XO tell you there are incoming reinforcements (the ones you already killed) and as soon as the alert finishes then mission finishes. They intended for us to fight them one at a time.

However, when you get the lighter lances all at once it's usually because of an ambush. They are meant to come at you all at once but are easier to deal with.

Not a bug, based on an AMA by one of the game's devs over on /r/battletechgame. They really shouldn't be called "reinforcements", according to him, but more like unexpected extra forces. To quote:

"Reinforcements appearing at start of missions confirmed not a bug, but it's unexpected due to poor communication. Likely changes to difficulty ratings of missions to make them more accurate".

reddit.com/r/battletechgame/comment/8g7dxm/hbs_developer_update_ama_summarized/