As already said, we don’t think it is related to DAVx⁵ itself. But reinstalling the app may clear some caches or the calendar storage which is somehow the cause of the problem. Full-day-events don’t have a timezone.
It seems that reinstalling DAVx5 is not necessary, at least in all cases. I had this issue a long time ago and now again today with two of my calendars containing tasks. Clearing cache of the OpenTasks app (dmfs.tasks) made sync work great again.
Steps to do this: Navigate to the app info of OpenTasks (or presumably your other favourite tasks or calendar app), tap “Storage” and press the “Clear Cache” button. A follwoing sync of the calendars worked without any issue.