Addendum - I put a total of 26 rounds in the drum and am again having problems seating the mag in the frame.
20 rounds - no go
15 rounds - nope
10 rounds - nope

8 rounds - success
9 rounds - success
It's as if the added tension from the drum spring is just too much to overcome. I am a sturdy 200+ pounder so it's not like I am limp wristing it.

I actually do feel a bit vindicated that the behavior I saw at the range was not just a fluke of some kind.
In case there is any question, this is a 1918 Erfurt frame I am using for testing.