The implicitly_retry_on_shard_transition_errors.js hook has no try/catch block to handle thrown errors - it assumes that errors are only returned as a command response. Other background hooks, like the retry_on_killed_session.js handle both error types.

It seems like FLE inserts are more likely to throw than normal operations due to the handling in the encrypted client, thus making it more likely a ShardNotFound error would escape.