They're not respawned, they are spawned on top of the monsters remaining from previous chapters.

wouldn't the strategy be, kill everything from first map before starting ch2 so they spawn back?

They spawn regardless. If a wolf survives chapter one, then he'll get a friend (or two) in chapter two.

Only way you'll miss out on any experience is if the game spawns two different species of monsters and they kill each other.

Oh, thank, then I'm good

All G. If you do a perfectionist play-through, you can rely on a source like this:

https://gothic2.maprealm.net/#/. Many gifted nerds worked on this (see credits).

For example, select "NPCs by chapter" and "2" while all other chapters are un-selected. In CH2, only 58 mobs are added to the Khorinis area. (The mobs you can't find have spawn conditions beyond the chapter number, i.e. be member of a specific guild or quest-related).

This may create a little bit of in-fighting among unfriendly monsters like the wolves and rats, but ultimately, not lose you enough exp to justify an undo on several hours of effort. In Gothic games, you clean the map in bits and pieces, exactly as you did; it's normal to leave some tougher areas for much later.

引用自 Яadu-TB：

Oh, thank, then I'm good

Yes :) Only the forests are full :)

It is more fun to play a game like this as a real RPG, it is no problem to skip things. All this 100% plus all achievements takes away some fun. It's not supposed to be work.