It behaves randomly despite whether I reboot the MacBook Pro or not. Though it seems I found the pattern. When I use my Beoplay H9 headphones at my desk in the office it shows AAC. But when I try it at home or in coffee shop it shows AptX (and the quality of sound improves significantly).
Most of the time I use 5Ghz WiFi (FYI just in case Bluetooth and WiFi modules are on the same chip). Also there might be some radio noise at 2.4Ghz in the office so the system chooses AAC over AptX.
I have the latest Bluetooth Explorer and it crashes every time exactly at the moment I open Audio Options window.
The reason of crash:
"Application Specific Information:
Performing @selector(createWindow:) from sender NSMenuItem 0x600000c3cfc0
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Modifications to the layout engine must not be performed from a background thread after it has been accessed from the main thread.'
terminating with uncaught exception of type NSException
abort() called"
I tried to reinstall the Bluetooth Explorer and XCode. No luck so far.
The Apple Support replied to my inquiry. They say that the AptX codec support in macOS Catalina 10.15.2 is partially workable and Apple's engineers are on it to fix it.