Add users ad-hoc to a meeting

We have users in Meetings First mode.  When in a meeting, if they try to add someone add-hoc after the meeting started using the Invite someone option in the participant view, this does not work.  I'm assuming this is because it's trying to dial-out to the user or direct chat and since we aren't in Teams only mode, this does not work.  Can someone confirm?  Do you need to be in Teams only mode to be able to add a person, room, etc after ad-hoc after a meeting started?