"Allow multiple selections" toggle keeps on switching off - user profile block

The “Allow multiple selections” toggle keeps on switching off on its own for no reason, to the frustration of my users who need to select multiple fields here (user profile block).

Does anyone know why this happens? It seems to not happen when I toggle off “Sync options with data source”, but I need that turned on.

The same thing keeps happening over and over again. Is there any solution?