“It’s not a bug, it’s a feature” …
This was a bit hidden, and frankly I did not expect such a feature (as default). Also it works not quite the way the setting in the app describes it: (translated by me from German)
Appointments older than 90 days in the past will be ignored
To ignore something means for me, not to do something with it. But DAVx⁵ does indeed virtually delete the local entries. This was not expected if I interpret the option as described and was kind of misleading.
The online documentation though does a better job and clearly states that older entries “won’t show up in the calendar anymore”.
Thanks for pointing me (again) to this setting. I was really concerned at first that my appointments are deleted, or that there is an underlying Android/Samsung issue. Sorry for reporting an issue unnecessarily.