Retrying the setAllowMigrations command with a session and a txnNum (retryableWrite:true) may end up with a deadlock, competing for a session checkout and the _kChunkOpLock acquisition, as it's shown below:

First cmd: session checkout > lock _kChunkOpLock > session check-in > transaction happens > session checkout > unlock _kChunkOpLock
Second cmd: session checkout > lock _kChunkOpLock > session check-in > transaction happens > session checkout > unlock _kChunkOpLock