This bug is allowing any board editor (team member, visitor, or guest) to be able to both hide and unhide and frame on the board via the frames sidebar.
In addition, any viewer or commentor on a board can briefly unhide a hidden frame by clicking on it in the frame sidebar.
I have opened a support ticket with Miro (ticket #536313) and they have acknowledged this bug and are working to resolve it.
I will update this post as I hear back from Miro support.
Update #1 from Miro support:
This issue was resolved.