Maybe bug?
I think if you cum too much at once in a single climax, the game doesn't give you cumflation skill points for her, like it's skipping over whatever's supposed to be the trigger that gives you the points.
(EDIT: just to be sure I checked a few different people. I don't know if it's the crazy cum volume or not though since the game still has them say the usual cumflation lines about being too full or being surprised at how much cum there is, but the stat itself doesn't seem to be going up.)
(EDIT2: dunno if it matters but I tested both DX11 and the normal one. Also with fresh folders and fresh saves.)