Hey Mike, lets get this figured out. This has happened to many people and I'm sure collectively we can figure this out and eliminate it.
I've never had this problem, albeit I've only been painting for ~2 years. I paint the figures with acrylic paints (including white) and seal them with delta ceramcoat semi-gloss indoor/outdoor varnish (brush on polyurathane). Then I decal and use I use the same delta ceramcoat polyurathane coating, this time matte for the uniforms and gloss for the helmets. The white on my teams from ~ 2 years is like the white of fresh fallen snow (maybe you don't know what that looks like down in TX
). The polyurathane coating is very DURABLE! I started using this from the beginning off of recommendations on the Miggle site from MGUTT, and I'm very happy with it.
Do use modge podge over decals, that could be the culprit. Also heard from others that Testors flatte spray coat may yellow over time. Maybe you can share your sealing method so we can learn from this collectively.
Joe