List of Users property doesn't work well when unknown user is recorded

Hi,

I created MyClass/MySheet/MyTemplate with some List of Users properties (NOT Multiple Select) and I have some strange behavior.

I create a new page (with MyClass object), I type "unknow user one" in my field, save my page and reopen it and type/save "unknow user two".

View and edit mode doesn't show the same: 2 or last value (see screenshot).
Property value contains "unknow user one,unknow user two"

Same behavior if I mixing existant user and unknown user: previous value is not removed.

(if I type existant user, previous value is correctly removed)

Thxs.

Pascal B