V3: very different card counts on different devices

I am using Anki Desktop Version 2.1.49 and regularly downgrading (via downgrade&quit) to Anki Version 2.1.43 because of an Addon that doesn’t work on 2.1.49. So whenever I create Notes, I use the downgraded version and whenever I review my cards, I use 2.1.49 with v3 scheduler.
I am using Anki on iOS version 2.0.83, V3 enabled.

Somehow my card counts have started to differ on both my Macs and on my iPhone. I rebuilt database, restarted Anki, disabled all Addons etc., but card counts still differ. And obviously I synced via standard sync.
And I am not saying, the numbers on the deck screen differ from the number shown at the bottom when hitting “study now”. I am saying that the numbers on the deck screen differ depending on which device I use. I usually use one specific Mac, so I was shocked when I once saw the higher card count on my iPhone and even more confused when I looked at the other Mac.
On the screenshots you will see one specific deck with two subdecks. But the problem occurs for more or less all of my decks.

Bildschirmfoto 2021-12-01 um 14.15.41

I suspect this is the same issue as Inconsistent card counts when syncing