Comment 16 for bug 1365769

you may need to open an upstream bug report at

https://bugzilla.kernel.org/enter_bug.cgi?product=Drivers

it is pulseaudio module-switch-on-port-available which change the input source when the jacks are not plugged

for the workaround, you may try disable Jack detection by hint using early patching

https://git.kernel.org/cgit/linux/kernel/git/tiwai/sound.git/tree/Documentation/sound/alsa/HD-Audio.txt

The generic parser supports the following hints:

- jack_detect (bool): specify whether the jack detection is available
  at all on this machine; default true