So obviously they all have different ORGANIZER-entries, which was no problem until recently and for that reason was not noticable at all for me as a user.
I can’t understand that, because I had this problem/feature (as you call it) myself a long time ago, and as I said, DAVdroid behavior didn’t change. However, I guess that your old events didn’t have any ORGANIZER, in which case the current DAVdroid account name is automatically used (by Android’s Contact Provider, not by DAVdroid!). So you might have not experienced any problems, because there were no different ORGANIZERs (but always the account name).
Theoretically, DAVdroid could override the ORGANIZER for events which are created/updated on the Android device and remove it or set something else. However, I didn’t see any reason to change Android’s default behavior.
Maybe you can mass-edit the events on your server (e.g. directly in the database).