That's an usual issue around here. I don't know if it's because devs don't upload the changelogs or it's a bug of the site.
However, the game specific forums normally have a thread with updated changelogs (sometimes updated by the developers, but most of the times courtesy of some other GOG users)

It's a regular thing and a pain in the butt.

This is one of the most annoying things here on GOG, because I want decent updates, but not pointless ones, especially when they are just about an installer update or language support etc. Particularly when not just a small patch file, and so it involves re-downloading the whole game, which could be many gigabytes.

In addition to the suggestion in the other reply, there is a dedicated topic for Updates, it is rather generic though and rarely tells you much ... does my head in the way it is organized, so I now ignore it.

https://www.gog.com/forum/general/the_what_did_just_update_thread

I wish GOG enforced Changelogs being kept up-to-date, but I guess they are between a rock and a hard place.

Good to know it's "normal", although it's quite annoying and to me it should be addressed urgently

I really wish GOG would remedy this.

Having to go to other sites just to learn what has been updated in a patch is frustrating!

That’s been like it for years:
https://www.gog.com/forum/general/provide_a_full_and_complete_changelogged_download_system

Unfortunately GOG do not care at all about it, or in fact anything else other than pushing galaxy. A lot of updates don’t even get flagged. Some updates break things or remove content, never to be fixed. That is why you pay more here than on other stores of course!

Case by case basis per game per developer.
This is a good way to spot a developer that appreciates goggers.
Even the best devs tend to forego putting the current patch notes in galaxy though.