Stuck input. Cursor on menu cycles. Whats causing this?

Probably the circle pad is not connected right

If you absolutely think that everything is correctly connected, then it must be the battery, replace the battery

Do u still have warranty or 30 day from where u bought it, if not, sorry but u will have to go send it to Nintendo BUT if u want it to be free, be sure everything is how the way it was originally, meaning all screws in their correct place, be sure that the screws are not badly stripped, obviously Nintendo would know u tampered with the system. If you added any tape or other things in the system, take them out. Good luck.

Did anyone figure out the cause of this?

I know this is kind of late but for me, I used a razor blade to cut off the d-pad adhesive on the motherboard and noticed some black marks on the d-pad contacts. I cleaned them with alcohol and put the adhesive sticker back and it was fixed. The adhesive sticker holds 4 metal dome-switches which provide the contact needed to tell the console you’re pushing a specific direction, and there were what looked like burn marks on all of the contacts.