Please check whether you're importing actuals for the _COST charge unit rate. These actuals shouldn't be imported, as Mosaic handles them internally through its billing routines.
If you haven’t imported actuals for the _COST unit type and you're still seeing issues, then please raise a case. When you do, make sure to include the Cycle ID where you're seeing the problem, along with an example person and their bill ID. This will help us investigate and resolve the issue more efficiently.