Both Call service + notify.mobile_app_xxxxxx and Device + Send a notification stopped working, worked just yesterday. The 500 limit is not reached yet, only 30 used today while testing it. The journalctl for HA says it sent the notifications but the app on android does not receive them. The android is a Samsung tablet with disabled power optimizations for HA but even if HA is open and running - there are no notifications displayed on the tabled.
How to fix this kind of problem? Notification to google hangout still work (but hangout is going to die soon though).