What is grammatically wrong in this sentence: Mum was washing the dishes while the match started.?

The match can only start at one time. So

Mum was washing the dishes when the match started. (ok)

The match cannot continue starting. "While" requires an event with duration. The start of the match does not have duration, but the match does, so:

Mum was washing the dishes while the match went on. (ok)

Should be: Mum was washing dishes when the match started.

Alternatively 'while the match was starting'. There's potentially more of a lilt to my suggestion, if you're into your Nabokov, I'd hit the imperfect running

Rhetorically speaking, what they are saying is that the sentence is syntactically flawed by a conflict of parallelism between independent and dependent clauses