I suspect it was undersized. Adding PC increased the size enough to fit properly. I will guess that you did not size them after application of the powder coat. If this is the case then the fliers could simply be bullets where the PC was not as thick.
Try the bullets as they come out of the mold and don't size them. Just lube and shoot. FWIW, the Lee molds were designed such that they would produce proper size bullets using WW alloy. For a couple decades I used the .45 bullets (RN, SWC and TC) without sizing. Cast, pan lube and fired.