I could open the issues in icognito mode. 1: Operation may block on prepare conflict while holding oplog slot, stalling replication indefinitely

2: Do not use the collection distributed lock for chunk merges

Yep. #2 is kind of a hush-hush way of them fixing a bug that can take a lock on a collection during a chunk merge and fail to release it, resulting in writes to primaries to stop until they are kill -9'd.