I have a fairly simple 2D-Geometry of a C-Clip base shape. Whenever I try to split this surface along its center line, a fresh cut line appears, but the surface doesn't actually split, in the sense that selecting one half automatically selects the entire part. This only happens when I split the part along y=0, other positions and orientations work just fine. In the example below, I created two cuts. The working one on top of the part split the rsulted in two separate surfs as expected, but the one along the center didn't, with just one (selectable) surface spanning over both halves of the part.
Currently on Hyperworks 2021.1. I think this is a bug, don't know if this also exists on newer versions.