As the title hinted, multiple games I bought on GOG don't show as owned when browsing the store.
Problematic titles are Pathfinder: Kingmaker and Fallout Bundle games.
For Fallout it's relatively straightforward - bought a bundle and got all the games no problem but they don't shown as owned when looking at individual listings in the store (also FO4 GOTY is seen as separate from FO4 + All DLCs GOTY on Steam but that's another topic).

For Pathfinder: Kingmaker it's so convoluted I'm not even sure which version i own unless I specifically go to the game's store page and swap to Imperial Edition:
- game's title is Pathfinder: Kingmaker - Enhanced Plus Edition but that's the only place where it's called that
- base edition is called Explorer Edition
- extra goodies edition is called Imperial Edition
- there's no easy-to-understand list or table of which edition contains what since the description has logo of regular enchanced edition which makes it easy to miss that it's about plus (feeling lost yet?)
- season pass bundle is not shown as owned but all it's content is indeed bought

I'm sure there are more examples I just haven't noticed them (yet).

This has been a known problem for a long time. Maybe some day they'll do something about it, but don't count in it.


edit: About Fallout ... here a a licence holder change happened.
Who bought the game before the change, still has it. but the new product is different from the old one, even if it's the same game. Different publisher, also different estras (the new version misses most of them).