Mueller upbraids Barr for misrepresenting the Report to the American people. Complains that Barr's intent was to undermine the significance of the investigation and the true findings. Barr's summary downplayed the import of the president's actions and how egregious they truly were. Mueller wanted to put out the Principle Conclusions as written and presented by the Committee, but Narr refused. Mueller submitted a letter, followed by a phone call to Barr to complain about Barr's summary and his spin.
Mueller is reported not testifying before congress because he still is employed by the DOJ and they have blocked his testimony. That will change soon but I question if that is not the reason that he has remained employed there this long.
Good question.