'<some number>' is not a valid number - caused by pressing Fn keys while typing command

If I press any Fn key (such as F2) while typing a command, (for example /tp 100 100 100) it makes the game return: '100' (in this case) is not a valid number.
This error persists if I copy+paste the command that I had pressed a Fn key while typing.

However, everything works as normal and the error does not occur if I type the same command from scratch without touching the Fn keys while in the chat.