I’m actually using Thunderbird too, and there the issue does not show up. Nor does it via a plugin for outlook 😕
EDIT: i installed another CalDav receiver (aCalDav - https://f-droid.org/repository/browse/?fdid=de.we.acaldav) next to yours on my phone. Seems to not trigger these issues.
I’d prefer to keep using yours, as it is a lot more refined, but right now, this bug is a showstopper for me.