I debugged https://salsa.debian.org/debian/devscripts/-/commit/e6b8179080ee087b2a2f3c349b19c6f541335b3b and in particular https://salsa.debian.org/debian/devscripts/-/blob/main/scripts/debchange.pl#L317-359 today.
I can confirm that this works as intended:
dch --no-multimaint-merge --append test
However this does not:
DEBCHANGE_MULTIMAINT_MERGE=no dch --append test
Unfortunately I didn't find the root cause yet.
Work continues in
https://salsa.debian.org/debian/devscripts/-/merge_requests/508
