I stumbled upon this problem myself.
Massprop seems to flip the sign of the product of inertia, or as someone else have pointed out, it rotates the reference system by 90°
You can try also with this minimal working example:
If you draw a square of length one (or any bi-symmetric shape) , the formula for the product of inertia can be simplified as follows:
Jxy = xG * yG *A
where:
Jxy: product of inertia
xG: x-coordinate of the square's centroid, i.e. distance from y-axis
yG: y-coordinate of the square's centroid, i.e. distance from x-axis
A: area of the square (=1 in this case)
Now, assuming the axes have origin in (0,0), put the square in each quadrant (counterclockwise) at this coordinates:
Quadrant I: (x,y) = (1,1) -> Jxy = 1 * 1 = 1
Quadrant II: (x,y) = (-1,1) -> Jxy =-1 * 1 = -1
Quadrant III: (x,y) = (-1,-1) -> Jxy =-1 *-1 = 1
Quadrant IV: (x,y) = (1,-1) -> Jxy = 1 *-1 = -1
Instead the Massprop command return the exact opposite for each quadrant.
