As opposed to what is implied in the changelog we had failures after adding shards to one of our cluster. When the balancer attempted to move 2.0 hosted chunks to a new 2.2 shard with the following errors :

"moveChunk failed to engage TO-shard in the data transfer: migrate already in progress"

The situation does not recover after restarts and is only resolved by downgrading back to 2.0 which immediately resolves the issue. Please change your release changelog accordingly.