Mueller was only authorized as special counsel to investigate Trump/Russia “collusion”. Anything referred to other offices or agencies would mean it has nothing to do with Russia.
Now, if the report were released and compromised another legal case, then Barr would be accused of causing that compromise intentionally. In addition, the report must be assessed by various agencies to determine if such release would impact national security in some way. Which is actually highly unlikely but look at some of the pure crap that is considered classified to hide improper behavior by our government, especially the “intelligence” agencies.