Hi @Wong,
Thanks for reporting this issue. This is understood to be an issue with the quality of the mesh. You are correct that it is due to the calculation of the center of the element being outside.
Please see
Drift-diffusion simulation for 3D produces higher currents than expected
for a discussion.
If you are seeing this in 2D, I am hoping the NodeVolume calculation is very close to the volume of the whole device structure. This may be due to limits of the precision, and I would be interested in seeing the differences you are observing.