Clear cached preload sitemap – php

How would it be possible to clear the sitemap preload cache?
I’ve noticed that when I remove the preload-sitemap folder contents

It’ll still work off a seperate file / cache?
The sitemap used for preload changes over time but it doesn’t change automatically fast enough for each preload pass by.

So i was trying to write a cron job for it but it’s not working as i’d expect, as the preload sitemap is cleared.
It’s still continues from the last point it was at.