Grouped user field - not imported & not possible to update using "wîth selected"

[rennecke]

I have the field "Unternehmer" which is of type "grouped user". It refers to a global list. The items there are linked to several users.
For a bunch of data (several hundred lines) I tried to import also this field. It was not imported.
Than I tried to update the missing values with "with selected" and "update" but I could not select values for that field. See picture. The "select an option" was already clicked.

[support]

Looks like not supported. Added to plan to fix.