I don’t know if it’s officially documented, or at least I couldn’t find it, but in general I question why a person would need to populate a user specific column when submitting a form. Generally if you are adding a new row that’s specific to a user, then either the entire row has row owners enable, or the data is filtered to only the signed in user. If the new row is available to everyone, then a user specific column could be populated with data from several different people, so most likely they would be editing the existing row.
I’m not saying that there aren’t unique situation where a someone would need to populate a user specific column when adding a row. I’m sure there are some situations where it’s needed. But in general, I would question why someone would want to populate a user specific column through a form.
Also, part of the limitation might be because user specific columns require a row ID. I think that row ID has to exist so user specific values can attach to the proper row. When submitting a form, a row id hasn’t been assigned yet, so user specific values would possibly be lost.