The Testors overcoats - both "Clear Lacquer" and "Clear(coat)", but particularly the latter - I have seen go yellow before, over top of white decals and paint (as well as other colors) alike, in 100% of cases (eventually), however never so quickly before. However, since I don't know exactly why it yellows in the first place, perhaps your batch was just "ready to go" (yellow), right out of the can. If you can, find out if your clearcoat(s) came from old stock - and test out some new ones, if you can get some.
And speaking of batches, that has got to be the explanation for your Hasegawa Tomcat decals - and only the Tomcat decals - being bad; no doubt they were all produced in a batch, and something(s) was done wrong. I would contact Hasegawa for replacements - specifically from another batch.
As for the aftermarket decals, if they are all from the same manufacturer I would suspect a bad batch there, too. However if from more than one then this can't be the reason.
In general, of course decals go bad with age but, assuming these are not that old, I would suspect any ageing environment in which you may have been storing them. Were all these bad decals stored together in the same place (that the good ones were not)? If so, consider any excesses of heat, humidity and/or exposure to chemical vapors - particularly anything acid, like acetic acid, which comes out of (silicone) weather-seal - or anything alkaline, like lime and/or concrete dust.
If all the above are eliminated; no bad batches of decals; decals not old; decals not stored in ageing environment - oh, and no odd/bad batches of paint underneath (which I have never seen, either) - then it would have to come down to the Tomcat kit plastic - again, very likely all from a single batch - which is releasing something through the Testors paint, to attack the decals.
In the latter case - which I must say, I've also never seen before - you could experiment with pre-washing the plastic, and/or use different paints (like you used to), under the exact same decals, and see if either one eliminates the problem.
A really tremendous cleaner/de-greaser that I would use for such tests is Chameleon® brand paint stripper; safe on every (except possibly old Nichimo) plastic - but you have to remove it thoroughly (w/soap & water), or it won't let any paint stick to the plastic, afterwards.
Anyway, the problem sounds serious enough to warrant some "lab testing"/observation, and I hope the above gives you some ideas, buddy.
Keep us posted.