left stick only moved up and down

Same here, I opened a brand new Ally rog and noticed this issue, so far don't know how to fix it.

Update:
I contacted ASUS service and was told it will take 1 or 2 months to get fixed due to no parts are available currently(in mainland China). I bought two ALPS RKJX21224001 stick controller and replaced both without broken the warranty seal. It's quite easy to open and replace the controller.

Found a reddit thread about this and it's actually just a small plastic thingy that got lose and is responsible for horizontal movement, just replace the stick or if you're feeling fancy open the stick up and arrange the plastic piece.

theres a little clip inside the analog module that moves the X-axis left/right and one for the Y-axis [up/down]. If you can pull off resetting the sensor in between the clips, you should be golden. It wasnt too difficult for me, being the amateur that I am, it only took me about an hour or so, i ended up doing both analogs eventually, as even the slightest softest drop can cause it to happen. Havent had to do it again tho!

obvious fix is you could just buy hall-effect sticks, theyre about $15 a piece if memory serves correctly

The reason is the bad quality of the stock sticks. Just get yourself some new ones from Amazon for example and with the guide here at ifixit swap the defective one out. It's a work of about 10 minutes and costs near to nothing. Fixed it for me and works ever since I replaced it.