I'm having trouble unfolding a very simple sheet metal part.

Context:  it is a stair tread for a curved stairway.

The feature Surface-Offset1 is taken from the assembly in which the Tread part exists.  Then I use the Split function wth the Surface-Offset1 to trim the ends off so that the tread mates perfectly with the sides of the stairway which are curved.

Here's the problem ... I try to place a Flat Pattern view into a drawing, and it fails, ie the view shows me the folded part. 

If I click the Flatten menubar button, I also get an error on the Flat-Pattern feature.  When I click What's Wrong? I get "The bend may be fused with other bend" .  (Is that even a cromulant sentence?)

When I suppress the Split feature, it folds fine.