I also share issues with Bluetooth since upgrading my 12 Pro Max to 14.4. It's specific to my car audio...
The vehicle is a 2016 Chevy SS, a.k.a Holden Commodore SS
The problem is that Audio playback from apps like Spotify or Youtube frequently skips. It's like the sound of a CD skipping. Interestingly, hands-free Bluetooth Audio calls no problem. Audio streaming to other devices such as my portable Bluetooth speaker and Bose headphones, Wife's BMW is all fine.
I have owned the iPhone 12 Pro Max since December 2020 and have experienced no issues until upgrading to IOS 14.4. I'm convinced this issue is specific to the new iPhone 12 family and IOS 14.4 because I can use my wife's iPhone XR (IOS 14.4) and my old iPhone 7 Plus (ios 14.2) in the same car without any skipping issues.
We know 14.4 brought changes to Bluetooth so that customers can specify what type of Bluetooth device is being paired. Could there be a relationship between this implementation and the problems being reported? Were there any other changes to Bluetooth such as chip-set firmware, buffering, signal strength etc.???
I have done everything, including a factory reset of my iPhone 12 PRo Max and installing the 14.5 Beta 2, hoping that it would solve the problem; however, both were unsuccessful. This is a very frustrating problem as I highly value the Bluetooth functionality in my car. I don't think it is fair to blame the vehicles audio system as it supports my households other iPhones just fine, and there have been no changes to it to explain this problem.