This problem has been present for me for years. At the moment in the side bar of installed games, all of them show up except for 2 games:

The Witcher 3: Wild Hunt GOTY Edition
Dracula 3 - The Path of the ...

Usually when I see this happen, somewhere between right away and a few days, I click on the missing icon and when the page loads for that game the icon refreshes itself and comes back. It remains there for a while although I'm not sure exactly how long. I estimate that it stays there for 2 weeks or so, but it could be longer than that. Either way, at some point in time in the future, if I have not clicked on the given game, it's icon will disappear until I click on it again.

It's important to note that with my currently installed list of games this is only happening to the two games listed above. The other games continue to show their icons just fine, including GWENT which I haven't played for 7 months, Shadow Warrior 2 which I haven't played or clicked on for 8 months, etc.

What makes this odd to me is that it seems to be more game dependent than user usage behaviour dependent. The Witcher 3 consistently loses its icon, while most of the other games do not. Very strange. I have not made any attempts to try to determine what other games might be affected by this as it would more or less require installing my entire game library and then leaving Galaxy sit there for some unknown amount of time on the order of weeks to months to wait for icons to vanish. I don't have the disk space available to embark on such an adventure so the two above can be added to a global list of "disappears randomly after a few weeks or more" if someone's keeping track in a list. If I discover any more over time I'll try to remember to report them also.

I should also note that this problem has been going on for a long long time now and uninstalling all games, uninstalling Galaxy, then blowing away the Galaxy ProgramData folder, remnants of the installation folder and more or less eradicating Galaxy from the system and then doing a fresh reinstall and installing the games, the problem comes back right away after a few weeks of not clicking on the game icons, but again, it does not appear to be random games but rather specific ones.

Perhaps the common factor lay in the goggame*.info files or some other meta files? I dunno, hopefully GOG figures it out though as it's kind of annoying. If they can't figure it out, then I hope they force the client to manually scan the icons of all games and force-load them instead. How hard can this be to fix?

Some obscure bug in icon caching? Simple, stop caching icons and just load them off disk every single time without caching. It's 2019, computers are fast, even those from 10 years ago are fast enough to not matter for this usage case. Or... fix the bug, whatever can make the problem go away sooner rather than later preferably as it just looks unprofessional and buggy this way. I've only got 11 games installed right now and it has been going on for a long time before I had this many installed. It doesn't seem related to how many games are installed but rather how long since a user actually clicked on the icon of a game that is already installed, and only then for certain specific games, not all games and not randomly. At least that's my 2+ year observation on 2 computers.

with me this only happens on my Windows XP machine(32 bit) and only with the Ultima series. other windows 7 and 10 machines they show up fine, all of them. I would rather not run Ulitma DOS games on windows 10 no sense in wasting such super heated gaming power rig on such things. have wanted to put those on windows xP machine instead. other than looking like "DOSBOX" icons it seems to run fine. I don't know if this ties in with the game's operation or merely a ICON art work problem wish they were there though they look soo fine!
strange though.
only thing to do (now) is search for another pic on windows limited selection or maybe draw something yourself and put that in instead.