For each activity that spans over a single updating period P6 recalculates the cost distribution for each one.
P6 do not remembers how progress was done on a per period basis unless you post it to a single pre-defined period.
Sorry P6 provides for a single Global Financial Period.
- You can work around such limitation by using separate database for each client with special needs.
- Get used to it, take it or leave it.
You have the option of moving to more advanced software that can remember progress per period as it was posted, per individual activity unlimited and variable duration periods that do not depend on a single pre defined Financial Period, to me even if a single Financial Period per Job it still runs short as even on a single Job the reporting needs of the different parties might vary.

A single activity that spans over any single period is enough to mess your global distribution. It can be the obvious long duration activities such as those for assigned of overhead, it can be the most unsuspected activity, even a 2 days duration activity can span over two periods.
WHAT A MESS, AND THIS BECAUSE OF A SINGLE ACTIVITY!