The screenshot shows what I'm talking about. This phenomenon (specific achievements missing from this page) is only appearing on Wingspan, but I can't figure out the corrupted file. I've uninstalled the game, followed by uninstalling Galaxy (and deleted gog folders on C: and in /localappdata/) to no avail.

It's no big deal if it's unfixable, but if there's a fix, I'd love to hear it.

thanks in advance

First thing is that Wingspan has invisible achievements. Contact support, they should set them to production mode.
All games with invisible achievements have exactly that problem with the horrible looking activity feed inside Galaxy at the moment.
As Ice_Mage already suspected, this is a server-side issue and directly related to the "no achievement boxes in profile" problem. GOG is aware of the issue and will hopefully fix this soon™. xD

Thanks for the help. I've got a ticket and we'll see.