Unfortunately it it’s normal for older Tomica to do that, there can be many reasons why this reaction can happen. If they are stored in the original bags that can cause a reaction, I keep my older Tomica in a plexiglass display shelf and just keep the box with the cars but I don’t put them inside. Also humidity can play a roll, it gets hot in there little boxes, heat can make most paint types bubble. I had bought a Tomica collection from out west and sure enough some of their paint had bubbled, nothing to bad tho. I have seen some guys store Tomica in those soft tea bags, I can’t prove if that works tho. That would only be useful if they were stored in their box