You say that is the only spot? Hope so, but you may need to wait longer to see.
And Id get rid of that section and an inch around it by knife. Quickly too!. It might be creeping.
Ok, My guess is you painted in more than one session, or began there. The reaction was your brush cleaner from a previous session was still not fully dry. Solvent caught in the heel, and under the ferrule, didn't evaporate. So the fist belly full of paint, was contaminated.(these are technical terms
)
If in doubt use primer first. It will seal it better, and may even allow you to spray the top coat.
I used green foam. Still doesn't like spray paint without primer, but the damage was immediate, then done. Not too severe really. (I tested)
I used left over Behr latex, on a non-layout sheet. No primer, no issue.
