Hi!
Alright, first of all I would clean the circuit membrane ribbon cable!
Clean it with isopropyl alcohol completely and see if that fixes it. If not, then it need to be replaced.
When it need to be replaced, make sure you have 100% the correct model! There are many of them.
Guide: [guide|142096|Sony DualSense Controller Buttons Replacement - iFixit Repair Guide|new_window=true] (till step 32, the circuit membrane is stuck to the black frame, that is the one you need to clean).
Please keep us posted!