Was that one working as a notification device?
Apparently it wasn’t. OP’s issue was solved after @ogiewon suggested testing from the device’s settings page, which then prompted a reinstall of the mobile app, and resolution of the failed notifications.
I feel that it's a bug in the Notification app, if it basically stops working if one of the notification devices stops working.
I'm not sure if it's a serious or significant bug, but I have personally found it difficult to debug because I couldn't easily see what was going wrong, just that there was no notification.
I've used the Notification app in the past for reminder of serious situations (leaks, doors not locked, windows left open, freezers left open, etc.), and to find out many months later that it stopped working because a notification device became unreachable is a frustrating situation.
