If I choose a Game, after updating, the newer Patches with changes and bugfixes are not shown.

Anyone else ?

It's up to the developer to do that per game.
Most don't for gog.

Are you referring to patch notes? To get that, you'll probably have to visit the site/social media for whatever game it is unless it's a CDPR game like the Witcher series and Cyberpunk.

So the Developers do this by Steam, etc. but not at GoG ?

Thats very sad :(

For the most part, GOG has yet to design a way for devs to do it outside of the forums, which not many check. It doesn't really hurt to go to their site, though. Sure, having it on the GOG site or on Galaxy is certainly a nice convenience but it's not a huge deal.

If you want patch notes, check out The "what did just update?" thread in the General Discussion forum. Use the search box to the left of the New Post button to find what you're looking for.