Some time ago, I purchased The Witcher 3 GOTY Edition.

As I understand it, that included everything Witcher 3 related (DLCs etc).

Yet when I check the pages of many of those elements, they are not showing up as owned. Same applies to that state being shown on Sales pages.

Take for instance, The Witcher 3: Wild Hunt (standard edition). That is surely part of The Game Of The Year Edition. As is The Witcher 3: Wild Hunt - Blood and Wine DLC and many others. Yet they don't show as Owned.

Are GOG aware of this and going to fix?

Aware? They should be. There are many games like this. Gonig to fix it? This has been a problem for more than half a decade. Maybe they're playing to long/slow burn method of fixing broken stuff on their site.

Well, I can't say I have noticed this exact issue with other games .... and I have a few.

And if there was one game you'd think they would get it right with, it would be a Witcher one.

Since you mentioned Witcher 3... The Goty version, marks as absent ("x") the comics extra, "reasons of state part 1"; in the store page. In the library, game's page, there seems to be in there, perfectly fine.

Other than that, i seem to have the same issue with "stasis". I had grabbed it free, back in the day, purchased the digital deluxe upgrade, but the "digital deluxe edition" isn't marked as "owned", for me (even though it technically is)!

Pillars of Eternity, Pillars 2, Tyranny, Pathfinder... Just to name a few.

You got the deluxe model of the game, it's not going to show you as owning the upgrades (DLC's) as they came with the game and you didn't purchase them individualy.

I also got the GOTY edition and fisnished both dlc after the main story

Well as I only have one of those and probably not the deluxe ........
But in any case I'm not discounting the possibility of others, just that I haven't recalled or noticed it for myself before. That is simply not true across the board, and is really just a matter of a GOG employee taking the time to flag things ... no magic involved.

Anyway, I am not here to argue, just alerting GOG in case somehow they missed it, plus an opportunity for others to also request it be fixed or mention other similar cases.