I have a 15.6" external monitor that allows 4 camera inputs. When I switch away from viewing the GH5s feed, then switch back to the GH5s, the monitor stays black until I shut the camera down and reboot it.
This does not occur with my DVX200, nor does it appear with my Canon 5D MKIII and MK II.
I know the GH4 had a bug where you had to boot the monitor before the camera or you got a black screen. Is this a similar problem with the GH5s? i.e. by switching the monitor away from the GH5s, then switching back, this could (in essence) play to the GH5s like the monitor was turned on second.