I need help to determine what will happen to my status miles in the following scenario.

Early next month my membership mileage will be c.31,200 miles leaving me c.6,800 miles short of *G for my next membership year. I am currently BD*G. This is after taking a flight in April that will earn me c.21,500 miles, while I'm still BD*G.

What will happen to the c.21,500 miles I earn in April - when I return to *S will these miles remain on my account or will they be wiped?