This sounds like a Lacquer vs Enamel reaction to me. Testor's has probably changed the formulation of their clear gloss coat. My recommendation would be to head to one of the big box hardware stores and by a rattle can of clear enamel gloss coat. If it does not specifically state enamel skip it. I recently got back into modeling and this is the approach I used. At various times I have been a painter as an occupation and I avoid mixing Lacquers and Enamels at all cost. I know I have seen people say on this web site and various others that you can get away with one over the other, but my real world experience tells me that sometimes you can and sometimes you can't and that may even come down to the colors of paint from the same manufacturer as far as to reacting with a different based paint being applied over them.
Besides a big rattle can of enamel gloss coat cost about the same as those little dinky Testor's cans.