Troubleshooting issue with strange key behavior

[the-mianaai]

Hi, I'm hoping for help in tracking down and fixing a problem with a V2 board. The PCB itself was ordered from JLCPCB and the parts from Mouser, but I haven't had any issues with any of the other boards in the batch (I made a few as gifts for friends)

The issue: Intermittently, when using the 3 key, the board will output 31 or 13; QMK Configurator board tester indicates a nearly equal time registering the two keycodes. This may or may not be accompanied by the 1 key not working at all. The person who was gifted the board reported similar issues with D triggering A, but I have not been able to reproduce that with extensive testing.

What I've tried:

• Reseating the microcontroller and checking for bent pins
• Swapping microcontrollers
• Reflashing the microcontroller
• replacing the diodes for those switches
• Visual inspection of the board for shorts, bridges, scratches
• Cleaning the PCB thoroughly to remove flux residue
• Reflowing the solder on the USB port
• Using a multimeter to test the R1-C2 and R1-C4 circuits while bridging the pads for the 1 or 3 keys

All of the above has either not fixed the problem or indicate that the board should be fine. My conclusion is that the most likely place for the problem is in the microcontroller socket, particularly since the C2 pin is directly in between the R1 and C4 pins, and somehow connecting the R1-C4 circuit creates a short inside the socket that also connects the R1-C2 circuit. I think this doesn't quite explain the issue with 1 not registering when pressed on its own, but I may be misunderstanding something. It also doesn't quite make sense to me why nothing else on C2 is having a problem (other than the reported C2/C4 and R3 problems that I haven't been able to reproduce, but the R3 pin is immediately next to the R1 pin, so maybe there is an issue, just a more intermittent one)

If this is the case, I'm wondering what my best options are. Desolder the socket and replace it (extremely not fun)? Disconnect the C2 and/or C4 pin(s), connect the traces to unused pins, and rewrite the firmware (not ideal as the person this was gifted to doesn't have extensive expertise with QMK and would have to rely on me for future firmware customization)

I'm open to all suggestions regarding possible causes for the problem and possible ways to fix it, and any assistance is greatly appreciated.

Edit: there definitely seems to be something awry with the matrix. It seems like R1C2 will get stuck/shorted open intermittently, and when that happens pressing R2C2 plus R1C3 sporadically returns both of those keycodes plus R2C3. Same thing occasionally happens between R3C2 plus R1C3 and R3C3