Noted. Moving on to the actual question.
Let me give you the actual state of things.
The Bluetooth toggle in Volumio governs the Bluetooth audio plugin, not the radio itself. Think of it as closing the app rather than switching on flight mode. The controller can remain powered, and if the host stack is still up, a phone can still see it and can still complete a pairing exchange. Nothing is registered to accept the audio connection afterwards, which is why the connection then refuses. That matches what what you described in 2022.
If you want genuine flight mode, block the radio:
sudo rfkill block bluetooth
Check the result:
rfkill list bluetooth
To reverse it:
sudo rfkill unblock bluetooth
Note that systemctl stop bluetooth is not equivalent. That stops bluetoothd only and leaves the controller powered.
I have a plugin in the works for Volumio 4 that would put this under proper control, but I am not going to commit to a timeline on it.
If you want the underlying behaviour looked at rather than worked around, post your board model, your exact Volumio version, and a log link from http://<your_volumio_ip>/dev, taken with the toggle set to off. Without that there is nothing to investigate.
Kind Regards,