Considering what we have learned so far I am not surprised to hear Mueller withheld evidence.
"Mike Flynn's attorneys said the government is hiding evidence that could help President Trump's former national security adviser.
Sidney Powell, the head of Flynn’s new legal team, claimed in court Tuesday the Justice Department withheld evidence related to Andrew McCabe, Peter Strzok, Lisa Page, James Comey, Flynn’s FBI interview notes, and more..."
https://www.washingtonexaminer.com/news/flynn-lawyers-claim-doj-withheld-evidence-to-force-guilty-plea