>>2044798
I explicitly said it in my post: the problem doesn't occur with the other, old, worn out derailleur - it's obviously not a frame issue.
Everything is the exact same but the derailleur.
The inertia of the forward-rotating cassette OVERCOMES the chain tension pivot spring of the shimano m4120 derailleur when starting to freewheel
The inertia of the forward-rotating cassette IS STOPPED SUCCESSFULLY the chain tension pivot spring of the sram gx 10s derailleur when starting to freewheel
The sram gx has a clutch, the shimano m4120 does not
I bet a shimano derailleur with clutch wouldn't have the problem either
Perhaps I could try a random old-ass shimano xt derailleur from the late 90s as third control to see if this is just a particularly shitty piece of shitmano shit

Either way, the brand makes nice stuff:
https://www.bionicon.de/

I got the cargo bike frameset for cheap but it's still a shame because it's still a nice product and effort went into making it.