Ah yes - I got many of my posts banned because of cursing with Jolla that they did not test properly.
This was the case for me as well - somehow it resolved after pairing and removing pairing and restarting bluetooth or killing hciattach in the console and restarting the phone. Once it was paired properly and worked it is now still doing fine after already 2 months.
I recently found out that on the Xperia X with 4.3.0.12 a downgrade of bluez5 to 5.50 works more or less fine.
If you are skilled ssh to the console and watch the log file