"Prevent sharing a project within ... with other groups" fails to keep existing groups access

I was hoping to use the “Prevent sharing a project within [a group] with other groups” option on a group to lock down the ability of users to add additional groups to a project, but keeping those groups that already had permissions.

This seemed to be in keeping with the instructions next to that option that says “Groups that already have access to the project will continue to have access unless removed manually.”. However, it seems that when you check the option, all groups lose access immediately and can’t be seen in the group members.

Am I missing or misinterpreting something?