It is surprising a Clinton-appointee is making this demand...some may see ulterior motives and I admit to that default in most all cases...but I think in this case the judge is actually trying to do his constitutional duty...and if the FBI can be made to comply with the request it will prove their perjury and unfaithful actions, that Flynn's plea was in effect fraudulent and coerced by corrupt means and could very well have his sentence vacated and the case thrown out. This gets to the heart of the greater conspiracy to stage a coup against a duly elected sitting President and it would be nothing short of explosive.
https://theconservativetreehouse.com/2019/05/16/news-in-flynn-case-documents-released-and-judge-sullivan-has-well-founded-suspicions/