In the file below, in Front view, call CutPlane and snap to each end of the red line. Normally that should create a plane surface that covers the entire section (so that you can split the object with the plane). In this case it does not get the extents in either the X nor the Y direction. Doesn’t work correctly either from Right or Top views. The object does not appear to be bad, exploding, rebuild edges and rejoining doesn’t help.
Tested in V6 and latest WIP.
BadCutPlane.3dm (171.9 KB)