[Motorola One Vision Questions & Answers] obs3rv:
So I tested a factory reset with the September firmware (PSA29.160-50), immediately trying to connect the headset (CSR8645) and the AAC-capable receiver (CSR8670), and the firmware consistently had the Bluetooth stack crash upon connection to both, both of which do AAC and AptX. So at least in my case, with my Bluetooth audio devices, the cause had nothing to do with any customizations I may have done, but rather reflected a real bug in the firmware.

However, upgrading to this month's PSAS29.160-50-2 permits the AAC-capable Bluetooth audio receiver (CSR8670) to work (without a factory reset, further providing evidence that the issue was/remains a bug in the firmware, and not anything I've done in Developer options or otherwise). The headset (CSR8645) still induces a crash in the Bluetooth firmware. The stack trace still looks like it may have the same root cause, so it seems a partial fix made it in that permits some devices to work but not all.

The AAC-incapable receiver (CSR8635) continues to work without problems.

Exception stack trace that causes the Bluetooth stack to crash:

10-30 21:07:06.581 24816 24816 E AndroidRuntime: FATAL EXCEPTION: main
10-30 21:07:06.581 24816 24816 E AndroidRuntime: Process: com.android.bluetooth, PID: 24816
10-30 21:07:06.581 24816 24816 E AndroidRuntime: java.lang.RuntimeException: Error receiving broadcast Intent { act=android.bluetooth.a2dp.profile.action.CONNECTION_STATE_CHANGED flg=0x5000010 (has extras) } in com.android.bluetooth.a2dp.A2dpService$ConnectionStateChangedReceiver@ebaeb45
10-30 21:07:06.581 24816 24816 E AndroidRuntime:	at android.app.LoadedApk$ReceiverDispatcher$Args.lambda$getRunnable$0(LoadedApk.java:1410)
10-30 21:07:06.581 24816 24816 E AndroidRuntime:	at android.app.-$$Lambda$LoadedApk$ReceiverDispatcher$Args$_BumDX2UKsnxLVrE6UJsJZkotuA.run(Unknown Source:2)
10-30 21:07:06.581 24816 24816 E AndroidRuntime:	at android.os.Handler.handleCallback(Handler.java:873)
10-30 21:07:06.581 24816 24816 E AndroidRuntime:	at android.os.Handler.dispatchMessage(Handler.java:99)
10-30 21:07:06.581 24816 24816 E AndroidRuntime:	at android.os.Looper.loop(Looper.java:220)
10-30 21:07:06.581 24816 24816 E AndroidRuntime:	at android.app.ActivityThread.main(ActivityThread.java:6929)
10-30 21:07:06.581 24816 24816 E AndroidRuntime:	at java.lang.reflect.Method.invoke(Native Method)
10-30 21:07:06.581 24816 24816 E AndroidRuntime:	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
10-30 21:07:06.581 24816 24816 E AndroidRuntime:	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:870)
10-30 21:07:06.581 24816 24816 E AndroidRuntime: Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.bluetooth.BluetoothCodecConfig.isMandatoryCodec()' on a null object reference
10-30 21:07:06.581 24816 24816 E AndroidRuntime:	at com.android.bluetooth.a2dp.A2dpCodecConfig.enableOptionalCodecs(A2dpCodecConfig.java:70)
10-30 21:07:06.581 24816 24816 E AndroidRuntime:	at com.android.bluetooth.a2dp.A2dpService.enableOptionalCodecs(A2dpService.java:702)
10-30 21:07:06.581 24816 24816 E AndroidRuntime:	at com.android.bluetooth.a2dp.A2dpService.updateOptionalCodecsSupport(A2dpService.java:953)
10-30 21:07:06.581 24816 24816 E AndroidRuntime:	at com.android.bluetooth.a2dp.A2dpService.connectionStateChanged(A2dpService.java:969)
10-30 21:07:06.581 24816 24816 E AndroidRuntime:	at com.android.bluetooth.a2dp.A2dpService.access$200(A2dpService.java:57)
10-30 21:07:06.581 24816 24816 E AndroidRuntime:	at com.android.bluetooth.a2dp.A2dpService$ConnectionStateChangedReceiver.onReceive(A2dpService.java:1007)
10-30 21:07:06.581 24816 24816 E AndroidRuntime:	at android.app.LoadedApk$ReceiverDispatcher$Args.lambda$getRunnable$0(LoadedApk.java:1400)
10-30 21:07:06.581 24816 24816 E AndroidRuntime:	... 8 more

I've updated Motorola as well with logcats of the successful connection to the AAC-capable receiver (CSR8670) and the repeated crashes attempting to connect to the headset (CSR8645).