Can't type certain letters after other letters in text box

When typing commands or other things in the chatbox, I get some weird results sometimes. When I want to type "/gamemode 1", I can type "/" and "g", but when I type "a", the typed letter does not appear. Instead, "/g" get selected somehow. When I backspace, "/g" gets deleted. Typing "/gA" is just fine.
If more examples or more information are needed, I'll add it.