when using the home-mcron-service, PATH is set to /run/current-system/profile/bin . This directory is empty when using guix home on a foreign distro, meaning all executable paths would need to be absolute. This includes stuff like /usr/bin/ssh, /usr/bin/nice etc..
My guess for the culprit was 1c30d5a6bfc5d48137f4bdcc271189a06fdc6ed3 , which replaced the custom home-mcron-service-type with mapping it to mcron-service-type. 
The mcron shepherd service in old service type did not mess with the environment variables, the inherited one does:
(remove (cut string-prefix? "PATH=" <>)
Strange thing is that the commit is from 2023-08-06, and I update guix almost every day, but did not run into the issue until today. But the commit seems to fit the issue perfectly, so I'm not sure what's going on at all.