I would call it a bug of the symbolic MuPad or its integration into Mathcad. Or at least is a very embarrassing debacle that the simplification can't be done with e, while it can perfectly be done with any variable (as you had shown with e) and also with the constants c or pi.
I see no reason why it should not work with constant e. I tend to call it a bug (which is also present in Mathcad 15. Guess that Mathcad 11 does not suffer from this.
