Upon re-examination I believe the TMP intermix "tower" wasn't a "floor" painting but rather some large canvas. When they rolled it out again for TWOK you can notice wrinkles "on" the walls of the level below
It looks like it was rolled out over a backlit transparency sharing the pulsating plasma energy effect of the upper tower structure (i.e. that part of the canvas was cut out).
Nevertheless, they seemed to have lighting issues and further needed to illuminate the canvas with lights, apparently hidden by the two ladies looking at the impulse engine fuel flow display. You can notice the shadow the physical intermix tower set element casts "towards" Scotty
Probably some light source at the bottom of the set for the intermix shaft visual effect needed to be hidden, hence this solution?