as an update, it's not just third-party apps, this problem affects even the "native" Apple Screen Time "App Limit" feature; Reddit is full of threads of people complaining about this starting with iOS 26.2

iOS 26.3 beta 3 might have a new (arguably worse?) behavior where instead of all limits blocking instantly on DEVICE pickup, each limit blocks instantly separately only when you open the app so if you have two separate limits (YouTube and Instagram), YouTube will block when its opened for the first time while leaving Instagram unaffected, until you open Instagram for the first time (where then that will also instantly block)

iOS 26.3 RC does not seem to fix the problem