Arduino/Mosfet control of CMOS logic chip
So the buttons were multiplexed. I identified the multiplexing outputs, and discovered that these signals went from -5v pulses to -1v pulses whenever the arduino was connected to the gates of the MOSFETs, even if it wasn't powered on. While I didn't discover why this was the case or why it caused "random" button presses, I managed to solve the problem by putting 1N4148 Schottkey diodes between the digital pins of the arduino and the gates of the MOSFETs. Now the mosfets behave as individual "buttons" and do not trigger unwanted events. Thanks for all the help.

edit: if anyone finds this as they're trying to olve a similar issue, feel free to message me and I can give you a full schematic of my solution.