Why is my left joystick malfunctioning?

Same thing happening to mine too. I went to open up my controller using my laptop screwdrivers and found a white residue around the board and solder where the button sensors touch. I cleaned with a toothbrush and alcohol. This fixed the problem but only for about 2 hours.

The buttons that were affected, were consistent with the game I was playing. Example: Pokemon Snap I would use the R trigger and C stick more than any other buttons, so those had the residue and malfunction. I am tempted to use my soldering iron but I rather do more research before I do any more self-repair.

I hope this helps a little.