Xbox library integration does not import Microsoft Store Games
Here is a thread about it, a few people reported this issue : https://www.reddit.com/r/playnite/comments/18tdqa7/fastest_way_to_add_a_microsoft_store_game_that/
I have a few games purchased via the Microsoft Store, mostly free games from Amazon Prime : https://gaming.amazon.com/
In order to get these games, the user needs to redeem a code here : https://support.microsoft.com/en-us/account-billing/redeem-a-gift-card-or-code-to-your-microsoft-account-d6b2c675-9e31-f312-7ff4-21834ad22fa8
Once redeemed, these games are shown in Xbox PC app but they are not imported by Playnite. It seems GOG Galaxy is able to import these but I've not tested it.
You need to launch Xbox/GamePass games at least once otherwise the API Xbox plugin uses won't "see" them.
https://github.com/JosefNemec/PlayniteExtensions/wiki/Xbox-Library-troubleshooting#not-all-games-are-imported
That's really annoying. Per the linked Reddit thread, GOG can see them just fine.