@makabaka1880 yeah, that's helpful, so I can get a reply in next 2 years that they're working on it. I'd fix it for them if they would let me :D

After 2 years, the situation is still not fixed. I've contacted Apple several times, to no avail. The warning I'm getting on macOS when I click "Mark as Watched" is: error 20:16:13.320478+0200 Developer Quota error saving (<private>)): Error Domain=SyncedDefaults Code=6666 "Exceeded maximum number of keys (1024) in store (9JA89QQLNQ.developer.apple.wwdc)." UserInfo={NSLocalizedDescription=Exceeded maximum number of keys (1024) in store (9JA89QQLNQ.developer.apple.wwdc).} Does anyone have an idea how to get around it?