There was a failure where mongod is stuck on waitUntilPreparedUnitOfWorkCommitsOrAborts as it is waiting for a prepared transaction to commit.

From this comment, the prepared update has txnid = 2019 and flags = 8 (WT_UPDATE_PREPARE_RESTORED_FROM_DS).

From this comment, the prepared update (on the index) has also a txnid = 2019 but its flags = 9 (WT_UPDATE_PREPARE_RESTORED_FROM_DS, 
WT_UPDATE_DS).
 
According to the dump, there is no session with txn.id = 2019 which means the transaction is committed. Somehow the update does not reach the disk.