Just in case that anybody still is interested in Mathcad bugs:
This was done in the current release of Mathcad 15. I guess we would see the same bug in Prime.
Its likely that the bug will not show up in Mathcad 11 (with Maple as symbolic engine instead of the current old version of muPad).
Glad to see that you are still around!
Its a strange bug. In the past most bugs with definite integral originated in the fact that the symbolics first solved the indefinite integral and made some assumptions which did not apply concerning the limits. But this one seems to be different.