Only conjecture.

a chance the dropout was partially masked for reasons unknown prior to the chrome layer.
Chrome does not like to plate at inclusive regions. Buffer operators don’t like to have frames get snagged and go flying.
of course lots of opportunities for that at the cable stop.

I have an Italian frame where the clamp faces of the dropout are showing chrome, chainstay shows chrome as does the aft edge of the dropouts, the rest of the dropout has paint.
terrible chrome plating too. No copper layer. It was a race bike. Color almost lifts with a heavy breath. Italy in the 1980’s.