Origins of the 405 line system.

[Mike Watterson]

Lift was not controlled on the cameras individually in the M-EMI system.  There was a master lift control very late in the signal chain.  The racks operators (who were actually engineers, and sat at the racks i.e. bays of equipment in the Apparatus Room) controlled tilt, bend and gain. Lift was done by another engineer - and all this is in Birkenshaw.

This might be the clue as to why the vision mixer couldn't cut.  The resulting instantaneous change in lift would be impossible to correct for.

That sounds like a good explanation. Perhaps later they added it per camera, maybe on a slightly later camera tube.