The bug:
If the info which gamemode you were in when you last entered a worldsave is not available/lost, upon entering this world and trying to switch gamemode with F3+N, the error is shown in chat: "Unknown command /gamemode<--HERE]"

As soon as you changed the gamemode via typing it into the chat, e.g. /gamemode survival, switching with F3+N is possible again.

This bug occurred when I entered an old worldsave with 1.16.2 release in which I haven't been since 1.14.2 release.