Upps, you r right. Delete parings. Pair and reboot. Then no auto connect?
Must look which BT chipset is in the dongle. I'm switched since 3 month to the RPi4 and there i doesn't need the external BT dongle anymore. All fine with the internal Bluetooth now.
I tried that. Delete it and redo it. It didn't work.
Under Linux Ubuntu/Mint I often had problems with some chipsets. The one I had now was running reliably under Mint. Maybe the RPi3/LibreELCE does not like this one... 
Read here:
Works with Audio Profile Addon, two scripts and udev rules perfectly for me!
Doesn't matter if i use the internal RPi3/4 or an external USB BT dongle.
I read that. But if I understood it correctly the script only looks for the BT address of the device. If device XY is online then switch to the defined profile. If device XY is offline then switch back to the normal profile. Is this correct?
But if the device (my headphones) are not paired, which is my problem, the profile does not change?
At the moment I have to do this every time before. Settings -> LibreELCE -> Bluethoot -> pair. Only then it works.
As said before with the BT of the RP3 this worked. What kind of BT-Stick do you have, what brand/type?
Hello again.
With the BT Dongle the sound is definitely not delayed anymore. That's great.
What doesn't work anymore is that my BT devices pair automatically. I've added them and trusted them. For example, when I start LibreELEC, they don't connect when I turn them on.
I just found a bad workaround:
I created /storage/.config/autostart.sh
#!/bin/bash
(
echo -e "connect xx:xx:xx:xx:xx:xx\nexit" | bluetoothctl
) &This only works if the headphones are already in pairing mode at startup.
Is there a nicer solution? Is it because of my BT-Dongle? With the internal BT the pairing worked without problems.
How to disable onboard WiFi and Bluetooth on Raspberry Pi 3
After edit the config.txt reboot with your BT stick plugged-in to LibreElec. Don't disable BT in the LE options!
works, Thanks. 
I has the same audio glitches/async/drop offs on my RPi3 with LibreElec 18. I disabled the internal Bluetooth chip and using a modern USB Bluetooth v5 stick. Since no more problems with audio!
I have the same problem. I have now got a BT USB stick. I deactivated the internal. How do I get the stick to work? Does it have to be installed? Sorry for the stupid question.