Well I am guessing here but I suspect what might be
the problem is the gas that comes off the paint
as it heats up.

Long ago someone mentioned this.  They recommended
that you take the back off of framed cels a couple
of times a year so the gases or whatever it was
that paint gives off can escape instead of warping
the surrounding plastic.

It kind of makes sense when you think about what
paint is made of and how it might might react when
it heats up and there is nowhere for the vapor to
escape.  Unlike a painting where the gas could just
go off into the air a framed cel traps the gas in
place which could damage the plastic or possibly
cause what you are seeing, maybe?  I'd suspect the
gas might be loosing the glue on the spacers so they
are sliding around.

Just a suggestion but perhaps you should open it
up and let it air out and if that works just do
that once a season or so.

Who knows at least 10% of that might even be true,
but I wouldn't bet on it  ^_^;;;