I;ve had an email from KF once when I asked about the excess miles when i hit gold.

Eg a flight gave me 6786 when I was at 49,000 miles and -> 55786 miles. The status would mention that you require a fresh 50,000 miles so back then I asked what happens to the excess 5,786 miles.

I got this reply ->

"The excess of 5786 Elite miles will not be brought forward to your next
qualification period.

Excess Elite miles accrued in previous qualification period will be one
of the qualifying factors for consideration should there be a shortfall
of Elite miles required for a membership renewal.

We hope this will give you some measure of assurance with regard to your
membership renewal."