Just tried it. You’re right, the alarm is not shown in the default calendar app’s event overview because Posteo sets alarm type EMAIL:SUMMARY:Test CATEGORIES:Keine UID:62cb616128ebbeddafecb73377256e10-DKeWgPW BEGIN:VALARM !!! ACTION:EMAIL !!! SUMMARY:Test ATTENDEE:mailto:email@example.com TRIGGER:-P0DT3600S END:VALARM END:VEVENT
So DAVdroid inserts an alarm with METHOD_EMAIL, which seems not to be shown in the event overview by some calendar apps, probably because it shouldn’t bring up an alert anyway.
However, in Android 7’s Google calendar app (which I have used for testing now), you can view/edit/delete the alarm when you open the event details. It’s just not shown in the overview.
So, you can eitheruse another alarm type (for instance, set the alarm with DAVdroid or another CalDAV client, or Posteo could implement that), or open event details and edit the reminder there, or you find a calendar app which allows to view/edit email alarms.
By the way, thanks to your report I just found that DAVdroid imported the Posteo alarms with an incorrect time (0 hours before the event instead of 1 hour before the event) because it had neglected the seconds. Should be fixed with ical4android/99ef17e1.