It is a well-known issue with probably less well-known solutions, cf. <https://unix.stackexchange.com/questions/723675/debian-ubunt...>
echo exit 101 > /usr/sbin/policy-rc.d
chmod +x /usr/sbin/policy-rc.d
I think this is the recommended way to avoid autostarting services on Debian.
Good pointer. I remember learning it, and then forgetting it. Probably more than once.
Still should be the default behavior.