Write Conflict using Mongodb 4.4.4

Our application has been working for 6 months fine using transactions on a single replica using Mongodb 4.2.
Installed version 4.4.4 this week and we are constantly getting:

WriteConflict error: this operation conflicted with another operation. Please retry your operation or multi-document transaction

Over the last 3 days we checked all our code to make sure we had not changed anything that would cause this.

Finally this morning I restarted version 4.2 server and found our application is working fine with transactions again.

Something has changed in version 4.4 and I cannot work out what has changed in this version that would case the write conflict. Searched the mongodb log file and there is nothing that shows a write conflict.

I have even installed 4.4.5RC and that version also creates the Write Conflict error.

Our application creates 2 connections, each connection is to database, creates a session with transactions enabled. The 2 connections are on the same thread.

Both databases has a transaction started.
Both databases are read, inserted and updated in their transaction.
Commit the first database and it does not error.
Commit the second database and it errors with WriteConflict.

It always happens on the second database commit.

We are using the latest Mongodb C driver 1.7.4.

Does anyone have any ideas why this is happening with v4.4 and not v4.2?
Do I need to change a setting?