Point Inside Closed Curve test not working

I have:

1) A series of 4-pt panels in space

2) A flat, closed-curve boundary on the Cplane.

I've set up my script to project each vertex onto the Cplane, and if any of those 4 points fall inside the boundary, that panel is culled out for further development. Fine.

Bizarrely, I have two panels (out of about 550) where a point is very obviously inside the boundary, yet my yellow Panel is registering them all as outside the boundary, therefore not being culled out. It is only the two panels, all the rest work as expected (including some in the exact same plane).

Is there a work around for rebuilding the point to register properly? Does anyone have any ideas on this? The point is simply not registering inside the boundary.