You're not alone. Since upgrading to 4.4.2 on a bone stock HTC One, I have had the same disconnect issues or better described, it constantly pairs and unpairs, and my contacts will now longer load, I have a 2012 BMW 328i with iDrive. I tried the latest firmware update for my iDrive, but the problems persist. A Google search shows scores of Nexus users who have upgraded to Kit Kat and have lost their bluetooth because of the exact same issues. However, I'm not seeing very many HTC One owners running 4.4.2 with Bluetooth connectivity issues under 4.4.2. I've counted only 5 thus far - you, me, and 3 others.
My wife's Galaxy Note 3 connects to my car with no issues, I've been looking , but have yet to found a solution.