Hello all,
it turned out that my problem was a result of an inconsistency in the
ACL variant (NFSv4 vs. POSIX1e) that existed in NetBSD-current for about
2 months. Christos was kind enough to look at it and fix it right
away[1]. My big thanks for that!
With all NetBSD-current builds with sources from 2021-11-27 and newer
the provisioning of an AD domain can be expected to works now. I tested
this with success with Samba from pkgsrc-2021Q3.
Many greetings
Matthias
[1] https://anonhg.netbsd.org/src/rev/21d465dbb2a8