Indeed, I just found out a few days ago that opening and closing the Type cover revived the cameras and touchscreen and it is inconsistent at best.
It seems like the Type cover open event is slow in triggering the Face camera because I often get stuck at the login screen where it is waiting for the Face Camera to initialise and doesn't go into the Pin screen like when the camera driver fails or is unloaded completely.
My SP7 is about a year old and I've never encounter this exact problem on Win10. At worst, it will fail to PIN if the camera fails to turn on. My 3 year old SP5 and its Type Cover doesn't exhibit this issue as well.
We know that the Windows Shell is slow in certain areas, so maybe it's a performance issue?