Assign an extra virtual lab manager to an existing vlab group does not work

Assigning an additional virtual lab manager to a vlab domain group that already exists (from the admin/domains/groups/ID view) does not work. The new vlab manager is briefly shown in the UI in the list of vlab managers, but refreshing the page reverts the change and looking at the outbound requests, no HTTP request is sent to the backend when clicking the Assign group manager role button.