There is a very specific combination of factors that must be fulfilled for this to happen:

1. Use mongodb-driver-sync 3.12.0 or 3.12.1
2. Connect with ?retryWrites=true

1. Do a write to a document with WriteConcern.UNACKNOWLEDGED. Driver will log that retryWrites=true cannot be used by unacknowledged.

e.g. 

userColl.withWriteConcern(WriteConcern.UNACKNOWLEDGED).updateMany(in("_id", ids), inc("impressionCount", 1));

db.user.updateMany({_id: [466]}, {$inc: {impressionCount: 1, {w: 0});}}

1. Do another update with Java for the same document, this time with writeConcern of 1.

db.user.updateOne( {_id: 466}, { $set: { 'email': 'hendy@hendyirawan.com'}}, {w: 1});

1. At this point the previous updateOne() is locked and will never return.
2. Also, pressing Ctrl+C does not terminate the app. Java app cannot exit. Doing a kill -9 will make connections stuck on the MongoDB server (at least on MongoDB Atlas M10 v4.2.2 it does).

Note that this bug is not reproducible when using mongo shell, so I think this bug is specific to MongoDB Java Driver. At first I thought it is due to codecs or thread safety or server busy, but even just updating a string reproduces the issue.

Workaround:

Don't use WriteConcern.UNACKNOWLEDGED (with ?retryWrites=true).  As long as I update always with w=1 or w=majority, there is no issue.