Green + red + yellow cause strum

Hi, having an issue where pressing the first three frets at once causes a downstrum. When I hold these frets, I can still strum up after the first downstrum has happened, but strumming down has no effect.

I have already resoldered the neck connector pins and have even gone as far as to completely unsolder the entire 14-pin strum bar connector from the "main" board, the one with the Xbox button. As far as I can tell, none of the connections on that 14-pin board are shorted to the one next to it and I've run out of ideas.

What could I do next to try and pinpoint the problem? Has anyone fixed an issue like this on their controller?