Joystick doesn't go all the way... Sometimes?

I noticed that sometimes, when holding left on my controller's joystick, my character would stop moving, or at least slow down considerably. I thought this could maybe be some sort of drift, but... It's kind of the opposite?

I took a video from hardwaretester.com/gamepad to better show what I've been going through.

For most of that video, I'm just holding the joystick fully left, with very slight up/down movements, but you'll see that it randomly loses intensity, and other than that, it doesn't seem to have any drift.

I found a guide teaching how to clean the sensors from a controller, and I was going to try that, but my controller is a 3rd party switch controller, and has been since discontinued... Only to say that I'm not entirely sure how to remove the motherboard from it in order to access the sticks, as it seems stuck somewhere.

However, when I opened it, I noticed these white spots on the motherboard, I don't know what these are, so I thought I'd share a picture of those too, in case they might be the cause.

I don't know what to do about this, is there any way to repair it? I rather liked this controller, it fit my hands perfectly, so I'm kinda sad to see it go even if it was rather cheap.