2018 yz65 fouling plugs all of a sudden

I bought this bike new at the end of 2018 and both my kids learned to ride it as the step up from the KTM 50. I think my daughter fouled 1 plug on her first day of riding. Other than that it has never fouled a plug

2 weeks ago my son was practicing starts and he would hold it open but when the gate dropped he'd chop the throttle which resulted in a couple fouled plugs which I can understand.

Today we were at a race meet, the bike was running fine, did practice and 2 motos then my son went to fire it up to head to the line for the last moto, it started he revved it a couple times and then died. I was the race before him and my wife didn't have time to change the plug before his moto. She kicked a few times, tried bump starting it but nothing. Another guy heard it and said it sounded like a fouled plug.

I've run the same jetting since new and went from 50:1 to 40:1 on the premix as he's starting to open it up more now. No spooge out the exhaust, no smoke once he's riding at race pace and the bike sounds crisp.
I will pull the carb apart tomorrow to have a look, at this stage I'm thinking float has moved and needs adjusting or the overflow hose is blocked on the carb.

Any other suggestions to what could be causing this?