How is F4311.PRRC Unit Cost calculated and recalculated?

I have a situation with a few Foreign currency purchase orders, where the customer currency unit costs are there, no problem, but somehow, at some stage in the order process, the Base currency cost was stripped out.

Checking F43119, it was there, and then it was gone, but different programs ran at that point, different order statuses, so no one common factor from that point of view. No one process or step that has caused it.

The only common factor was a year-end in between order entry and the PRRC being zeroed out, and some of the items (but not all affected) were repriced (so the amount changed in some cases, not in others) and it seems that the unit cost (PRRC) was somehow set to zero at the time of yearend.