Bubble in Slump
There are two things that seem relevant to my diagnosis on why raised areas appear on slumping:
The experience I have is with moulds that have the vent hole in the centre at the deepest part and also have vent holes on the side of the mould, making me certain that there is ventilation for the glass to push out the air as it falls. I always do a visual inspection of the vent holes before using, and looking at other’s with the bubble on slumping problem, they also have free holes.
The second is more theoretical. If you are slumping at such a temperature to seal the glass to the mould, you are firing too hot anyway. Or put more positively, a low temperature slump (that is, a slump at the lowest temperature to achieve the desired result over an extended period of your choice) A low temperature slump will allow the glass to conform to the shape of the mould without softening so much that it takes up all the markings of the mould. Therefore, there are spaces for the air to escape from under the glass all the way to the top.
It is high temperature slumping that causes most of the problem of creating a bubble at the central bottom part of the slumped glass. Of course, trapped air can be a problem also. But if you have a bubble with no trapped air possible, because the vents of the mould are open, you have to consider other possibilities.
These are the things that make me certain that not all bubbles are caused by trapped air.