The NARRATIVE you've absorbed is that he was hiding the letter, when in fact he made a public comment to address the letter the day after receiving it.
IT IS A NON-ISSUE as the letter was requesting the 19 pages of summaries Mueller wrote to be released in addition to Barr's 4 page summary of Mueller's conclusions. Instead, Barr released all 448 pages, far exceeding the desire of Mueller.
Mueller's letter did not question the veracity of Barr's conclusions. It addressed a discontent with the MEDIA -- NOT BARR.
Where are you getting your information?