I'm not a Tamiya acrylic expert, but nobody has responded so if I may toss out my 
then maybe the bump will be seen by some regular Tamiya users.
InquisitiveBinChicken
The base coats had been applied at least one week prior, so they should have dried.
I agree, I'd think one week should have been adequate, but....
InquisitiveBinChicken
I thin them with water.
I don't know the answer but I'd be suspect of the water. I've only used X-20A. Folks seem to thin with lacquer thinner with outstanding results.
My thought process is below. Reading further is optional. 
If one searches for 'can tamiya acrylics be thinned with water', there is a lot of typical anecdotal crap (like this post of mine!) and no real answer to the question. When I ran across posts by names I recognize as knowing something, the trend seems to be they don't recommend thinning with water.
I found a statement by Tamiya yesterday (which I can't find now, sorry) where they said one of the nice things about Tamiya acrylics is it can be thinned with water and you can use it to clean your airbrush. But for thinning we recommend X-20A thinner. (I paraphrased that) I couldn't figure out if they were saying it can be thinned with water or not.
It's my understanding that X-20A thinner is 91% ISO with touch of butanol which apparently serves as a drying retardant. The chemistry is above my pay grade, but I sure can't figure out how 91% ISO with a touch of butanol is somehow equivalent to water.
I hope someone with practical experience jumps in with a more definitive answer for you.
Edit: Tojo and I cross-posted. (I was unaware of his reply, so it looks like I ignored his but rather I didn't see it) I think his idea of a dissimilar barrier coat is excellent! Whatever caused the problem, I agree a barrier coat would most certainly prevented it. 