Write conflict in very simple Access form with subform

This one is stumping me because what I'm doing seems so straightforward.

I have an unbound form (frmAdmin) with a listbox (lstUsers) and a subform that also contains a listbox (lstUsergroups).

lstUsers is unbound and shows a list of usernames (row source is the users table left joined with the contacts table on users.fk__ContactID = contacts.pk__ContactID so I can get the names to display)

The subform is bound to the users table and has the filter pk__UserID = Forms!frmAdmin!lstUsers

lstUsergroups shows a list of usergroups as is bound to the fk__Usergroup field in the users table.

The only VBA involved is a click event on lstUsers that requerys the subform.

When I select a user from lstUsers the subform listbox shows the user's usergroup selected, but if I select a different usergroup and then select a different user or close the form I get the write conflict "This record has been changed by another user" etc.

This seems so simple! What could cause a write conflict? There are no other forms open, the users table is a linked SQL Server table but there are no triggers anything that should be writing to it. It's just a form, a subform, and a click event.

Make sure the SQL table has a primary key, then update your linked table. If that doesn't seem to resolve it, try adding a timestamp field to the SQL table that's getting the write conflict, then update the linked table again. This happens to me all the time. Access seems finicky about what it actually uses as unique identifiers sometimes.