Hey Chris,
Just made another video to show the particular area I was talking about.



Notice in the video that regular 2d "planar curves" will only have an X+Y value? so a curve that is non planar will show a Z value also (Its a very small value)

I have a program which will display out to 15 decimals and the Z number on those curves is "0.000065326690674"........

That number changes as you set decimals with rounding. So even the others you saw that said "0" in MoI, were out by some factor.

I think this is what was messing with the fillet calcs (or better said MoI's display mesher with bad trims) and why the STl still calced out. (Number that was out is VERY small)

Anyway, maybe look into why you got non-planar curves will help you to avoid this in the future...