I tested in a jammy schroot (with mmdebstrap 0.8.1-1):
$ apt install mmdebstrap qemu-user-static debian-archive-keyring $ mmdebstrap -v --variant=minbase --mode=unshare --architectures=armel --keyring=/usr/share/keyrings/debian-archive-keyring.gpg testing example.tar
and that succeeds.
I tested in a jammy schroot (with mmdebstrap 0.8.1-1):
$ apt install mmdebstrap qemu-user-static debian- archive- keyring =armel --keyring= /usr/share/ keyrings/ debian- archive- keyring. gpg testing example.tar
$ mmdebstrap -v --variant=minbase --mode=unshare --architectures
and that succeeds.