Since version 1.4 apt supports ASCII armored keyrings in
/etc/apt/trusted.gpg.d. If mmdebstrap is run in such an environment, the
following error is produced:

gpg: [don't know]: invalid packet (ctb=2d)
gpg: keydb_search_first failed: Invalid packet
E: gpg failed at /usr/bin/mmdebstrap line 170.
	main::error("gpg failed") called at /usr/bin/mmdebstrap line 4915
	main::main() called at /usr/bin/mmdebstrap line 5796

A workaround is to run mmdebstrap with manually passed keyring material.
But normal executions like:

    mmdebstrap bullseye chroot.tar

Will fail with above error.

This is fixed upstream in commit ccd4b5c1 and in Debian since version
0.8.0-1.