Thanks a lot to Firelord's response.
I followed the leads they gave and ended up finding the culprit is the smart lock I installed for my home that requires bluetooth.
I found this line in android.log file: BluetoothManagerService: enable(com.august.luna).
With OnePlus 6, I didn't have to use adb, I was able to:
turned on Developer Option,
turned on Debug ...
On my OnePlus 6 running OxygenOS 10.3.11 (Android 10) I can find out the culprit using the following adb commands:
dumpsys bluetooth_manager | toybox sed -ne '/abled.*due.*to.*APPLICATION_REQUEST.*/p'
$ adb shell
$ dumpsys bluetooth_manager | toybox sed -ne '/abled.*due.*to.*APPLICATION_REQUEST.*/p'
01-05 13:31:04 Enabled due to ...