systemd-container login as root no longer possible?

[arne <sp113438@telfort.nl>]

After upgrading my containers from stretch to buster I can no longer
log in as root

I make the container with debootstrap, then 

# systemd-nspawn --directory=/mnt/mycontainer passwd

then I start the container with

# systemd-nspawn --register=yes -D /mnt/mycontainer /sbin/init &

then

# machinectl login mycontainer

gives me a login screen

When I enter "root"
no password prompt is shown


In stretch containers there is no problem, I get the password prompt
and can log in as root.

I tried with buster to make a new container on buster, can not log in.

Perhaps it is a new feature of systemd?

Lucky me, I had copies of my stretch containers.

Is this a bug or am I missing something?