New information : if i create a new host named foo, in puppet classes smart class parameter, it adds the matcher fqdn=foo instead of fqdn=foo.domain. If i add the domain to the matcher when i edit the host values are then overrided.

So i solved the problem providing fqdn as name instead of shortname when creating a new host. Isn't this useless as we must provide domain in network tab ?
And when i edit the host after creation, display name is the shortname ... maybe caus i use the use_shortname_for_vms setting

i'm using a vmware compute resource.

I suspect this is due to this change:
https://github.com/theforeman/foreman/commit/4bae5cedb8ab562b6c397234c5cb1cfd604fde74#diff-2d53894341fd1cf84a823e3bd0855495L740

Stephen, do you remember why that was?

Confirmed that restoring the line fixes #4227. I suspect though adding it here might lead to multiple errors being added (line 777), and we need to check the other code above it is OK with being run multiple times.

What do you think about providing an fqdn getter to ensure we always return an fqdn?

Like this:
https://github.com/stbenjam/foreman/compare/4234-override-host-parameters

Figured I'd ask before opening the PR to see if you had a better idea in mind.