I've noticed this, but it always seemed to just be a sorting/updating issue. Some scenarios:
-If I'm browsing the site on my PC to add videos to my playlists, it usually takes awhile for videos added to the playlists on PC to show up on my profile on my Quest 2.
-If I add a video to the playlist from any source, it doesn't always add it to the first or last page, but if I choose a random sorting option and then go back to showing the most recently added videos, it'll now show up where it should be.
At least for me, the video has always been successfully added to the playlist, but the site has some finicky quirks and doesn't always display these changes properly or timely.