If there is only one user account not respecting setting for default-sink it may have accumulated some conflicting user configurations in PulseAudio settings databases
One simple method to reset PulseAudio databases is to delete the databases and cookie in user home folder and restart PulseAudio
rm ~/.config/pulse/*.tdb ~/.config/pulse/cookie; systemctl --user restart pulseaudio
I also suggest using this command to ensure the default-sink is set to the Bluetooth headset
pacmd set-default-sink bluez_sink.5C_FB_7C_F4_6B_DE.a2dp_sink
If that does not work as expected, use this command to suspend the ALSA sink when using Bluetooth headset:
pactl suspend-sink alsa_output.pci-0000_05_00.6.analog-stereo 1
and this command to de-suspend the sink to use onboard audio
pactl suspend-sink alsa_output.pci-0000_05_00.6.analog-stereo 0