Since I have installed Galaxy 2 I have lost my banished achievements. These were complete since ages, but for one reason Galaxy has deleted them, not once but twice!

Anyone else has seen this behaviour?

What do GoG Support say?

Failing a solution from them, try telling Galaxy not to automatically update the game.

How does the game remember achievements? Not in the Save folder. I never use Galaxy - I prefer to play my games offline - but I lost all mine when re-installing the game on a new desktop (the old one had died) despite bringing my old Save folder across. Does it use its Windows registry entry, which we cannot readily inspect, or does it somehow zap its own internals without Windows marking this with a new date-timestamp?

Yep. Galaxy seems to be only able to track achievements the moment they are done if Galaxy is running. Played a bit with an old save of mine today; in-game all achievements I unlocked are accounted for; Galaxy treated it as some game I barely touched.

Another game I played while Galaxy was off was Kingdom of Amalur; played a bit later with Galaxy on and it only tracked one achievement.

I personally don't really mind that. Its a kinda nice to see thing on occasion as a milestone tracker but overall... meh. They get interesting if they are ingame and grant some bonus (i.e. +1% on X for doing X 1000 times). But as an achievement hunter I guess I would be quite upset about Galaxy 2.

I know this post is old but I’m looking for the opposite, reset achievements and start anew.
I managed to delete all achievements by editing registry keys.

The achievements store in the following registry path.
Computer\HKEY_CURRENT_USER\Software\Shining Rock Software LLC\Banished
You will find there are Ach0, 1, 2, max at 35. There are 36 achievements in-game, start with index 0, which translate to Ach0 in registry.
Delete those keys means to delete the achievements.

If you want to cheat for an achievement, you have to create a proper registry value.
New > String Value: Name = AchX (X is a number)
Modify > Value data = true

The ordering is the same as in-game achievements window, the first is Ach0, the last is Ach35.
Replace X with the number you want.