Why is the notification coming 9 hours late?

I created a full-day event in my Google Calendar using my Android-phone. The notification is set to "7 AM on the same day". But for some reason, it notifies me consistently at 4 PM (4 minutes ago in the screenshot below). I did already delete the entry without effect. What can I do to get the notification at the time displayed? My timezone-settings seem to be correct: the phone displays the correct time and notifications for normal (non-full-day) events show up at the correct time. Also all events that are synchronized from my Exchange-account are displayed at the correct time. The device is a Hauwei P20 Pro with a stock-rom.