The circumstances under which this occurs are when a class are not available in the environment the `HostGroup` will be changed to.
If the target environment does have the class available it will be kept. This does not happen instantly (anymore) but requires the form to be submitted.

Since Foreman does not create classes in puppet, but only reflect what is in puppet, keeping the classes in host groups as they are could lead to confusion, if they are not actually in the current environment. Keeping them, and marking them as "unavailable" if the environment does not include it and allow to remove them manually.

A simpler approach would be to simple warn in case classes are not available and will be purged on submit.

Thoughts on which route would be the more feasible way to go?

I prefer warning about purging on submit.