If it's worthwhile, APDL could be used as a check on the values in Mechanical. I think it's possible that a small error in the centroid location could create a large error in the moment. The APDL commands would be something like this:
cmsel,s,face  ! face is a named selection for the top flange face
esln         ! is the tube connected by contact? If so, this command will only select the tube elements and contact elements
spoint,,x,y,z   ! x,y,z are the coordinates of the centroid
fsum        ! Compare with Mechanical
allsel
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