Humm...
It was definately - lacquer paint and lacquer thinner that got mixed together?
Not an enamel and a lacquer?
Lacquer should not be sprayed over enamel as it can have an ill effect (cracking, peeling, blistering, etc...).
What effect are you trying for? What was the semi-gloss coat... clear?
Lacquer paints dry somewhat dull and rough and need to be color sanded/polished smooth.
If the underlying surfaces still look fine, it's probably something
regarding the mixture of paint and thinner. Sounds like something
wasn't compatible with each other. Possibly a chemical reaction in the
fresh paint. Check the bottle of MM and see what thinner it recommends
using.