Some users have experienced stick drift issues with the Steam Deck. Posting videos on the subreddit r/SteamDeck, three users reported cursors moving on their own, both in-game and in the OS. Valve has now acknowledged the issue and deployed a fix.
https://www.notebookcheck.net/Valve-issues-a-speedy-update-as-reports-of-Steam-Deck-consoles-suffering-from-stick-drift-start-to-come-in.605340.0.html