Trimming Polyline in rotated UCS fails

I seem to have a knack for finding this stuff. Probably because I work in 3-D and most users are in 2-D. Rotate UCS 90deg on X-Axis. Create a rectangle. Draw a line from mid of the top line to mid of the bottom line. Trim using that new line as the cutting line. The trimmed polyline is created, but it is in the WCS.