Just to clarify, you see W typed in system boxes after closing minecraft? If so it points to a system issue.

Are you using any mods? First step would be to create a new instance in prism that is vanilla. Fire it up and see if it happens. If not, likely Mod related.

Bad lag from low memory will do that. Used to happen to me all the time before I tweaked a bunch of settings.

That's an odd issue.

Holding W shouldn't prevent the Esc key from being sent. It makes it sound like something happening in the hardware.

Are you using any gaming mice/keyboard that may have a macro enabled? For example, if you wanted to run forward for a long time and the game doesn't support autorun you could have a macro that held W which could be toggled on and off through a keyboard/mouse key.

I wouldn't worry about it too much since you were able to resolve it by rebooting. If it's something happening constantly then it would be worth troubleshooting, but generally you are wasting your time trying to find out what happened during a one-off issue.

If it were happpening to me, I'd wait for the issue to occur again and then SSH into the machine from elsewhere and run evtest, select the suspect keyboard, and see if it is emitting scancodes.

This will tell you if the problem is the hardware or something in the system. Seeing scancodes that are showing the W key would mean that the hardware is sending the keys, not seeing scancodes either means that the problem is your system OR that there is another device sending the scancodes (the other device will show up in evtest as well).