Why this is happened with my PBN?

In my very first PBN network, built years ago, I basically linked 2 money site. I know this is bad but still it's an old work.
After years, yesterday one of my two website linked there went from position 4 to second page in a night time. And it's not a dance because it's on page 2 today too.
But the other website linked in the PBN is still ranking fine in position 1.

Why this happened? What could be the reason?