@AndrewFerr invited me to a v12 room that was restricted to the Element internal space. Upon trying to join it, I received the following error in Element Web:

Unable to find a user which could issue an invite

This appears to be due to this code:

which checks to see if any of the room's creators are local users. In this case, the room creator was @fair:miscworks.net, which is not a local user.

Thus we drop out of the if create_event.room_version.msc4289_creator_power_enabled statement, past the if chosen_user and user_power_level >= invite_level as chosen_user is None, and hit the SynapseError.

It looks like in the case of v12+ rooms, this code should be re-worked to consider both room creators and any local user that has the power to invite other users.