In some cases, when you are pressing a key (in my example Key: 'A'), so that you go to left; and you're player switching the server, for example, thru a bungeecord or something similar (in my example: Hypixel-Game -> Hypixel-Lobby), it can be stuck with the key, without pressing it, after the server-switch/reconnect, so that you walk left infinite. (Until you press the actual key again)
Does this issue occur in a complete vanilla environment on the latest release (1.17.1)? You mentioned that you can reproduce this on hypixel, but this server is modified therefore invalidating this problem. Please let me know if you are able to do so. 😃