I am hesitant to make this a suggestion,,,,because I think the Ammonia is the cause of the problem
So, this is me just thinking out loud (or, out keyboard?) Maybe some Ammonia based cleaner will take that back off? I mention it because I have rejected every product suggestion in my "modeling lab" that includes Ammonia in it. I don't use Windex, I use a Chlorox version of simple green, the "home made thinner brews" for Acrylic paints that include ammonia, I either reject or modify the mixture.
one other possible substance that might cut that stuff after it has "dried" into a rubber ball,,,,,,is the Medea airbrush cleaner,,,,,,it can cut dried Acrylic paint out of an airbrush
Don't take any of this as a "how to",,,,,,it is just my thoughts on things that I know can sometimes ruin a perfectly good paint job,,,,,,which is sort of the same problem that mask has given you
oh,,,,,,,and I am sorry to read your post,,,,,,,,you must have a lot of work into your build if you are at the masking for later colors stage
Rex