Looks like a new version was rolled out, at least for the non subscription lab rats:
root@pmg1:~# dpkg -l|grep pmg-api
ii pmg-api 9.0.6 all Proxmox Mailgateway API Server...
Hi,
yes, for Proxmox VE 8 there was no backport of the changes yet. It's planned to be done once the improvements to enrollment land, so everything can be backported at the same time.