systemd version the issue has been seen with

systemd 241 (241.159-1-manjaro)
+PAM +AUDIT -SELINUX -IMA -APPARMOR +SMACK -SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD +IDN2 -IDN +PCRE2 default-hierarchy=hybrid

Used distribution

Manjaro

Expected behaviour you didn't see

machinectl start work_dev

Unexpected behaviour you saw

Invalid machine name: work_dev

Steps to reproduce the problem

1. Create a container in /var/lib/machines called work_dev.
2. Try to start it with machinectl start work_dev.

I don't know if this is related to #3788 or not, but after upgrading from systemd 239 to 241, machinectl stopped working with my container. My container is called work_dev and it worked fine up until I upgraded to systemd 241, now it says it's an "invalid machine name" because of the underscore.