I would think that the heat would be from the concentrated light emitted by the projector building up in a specific print area as opposed to the resin heating up ( which seems to be a good thing )
The glass would dissipate the heat between slices but eventually it would have an affect on that area exposed
makes sense even if you think about it in hours of intense heat in one area like turning the spot light on every 3 or 4 seconds eventually that area it is on will heat up over a 3 to 8 hour period of time.
The one factor that seems to be unnoticed here about Robert's longevity of PDMS is he has a computer fan placed on the side blowing across the lens of the projector to reduce heat. That alone may be the answer to longevity of pdms.
Robert shut off your fan see how long the PDMS lasts ( just kiddn)
anyway that is what I see as a difference here
I'm so far behind I think I'm First !!