I did mention I was just guessing.  But really I think
it isn't a matter of how much time the cel has been
framed but it is the heat it has been subjected to
which will determine how quickly or how much the paint
discharges as fumes  (you did mention this happening
on a particularily hot day that is where I got the
idea ... kind of like when you go to pump gas on a
summer day and you can see the fumes coming from the
nozzle even before you start pumping.)

Anyway it was just a thought.

As for the spacers I wasn't sure what kind were
used.  The cels I have that are framed were done
by professionals so I actually didn't get to see
what the spacers looked like.  I just got this
idea in my head that they would need some kind
of adhesive to attach to something.  I'm glad to
find that I was wrong about that.

Given that, I don't have a clue what is causing
the symptoms you are seeing...  sorry.

Good luck figuring it out.