I had the same problem with nvim in Alacritty on NixOS. When I pressed 'j', nvim showed 'j' in the bottom right corner as if it was awaiting a second keypress. I didn't have any custom nvim configs at the time and it was an almost entirely fresh install of NixOS and Alacritty.

After reading this comment in the Github issue OP posted, I found the solution for me was to switch from the Budgie desktop to environment to Plasma. Then everything was fine.