PR is de-released after change of purchasing group through MASS transaction

Dear All

Whenever I run MRP planned order is created.This planned order is then converted to PR.This PR is then released finally.Fiinal release indicator is having changeability option as "1" i.e. "Cannot be changed".After final release through MASS transaction purchasing group of PR is changed.After changing the purchasing group when I display the PR again it has completely de-released.Purchasing group is not part of my PR release strategy.

Why the PR has completely de-released after purchasing group change through MASS transaction???

Regards

Satish Kumar