In any text field in the game, the function keys count as a character. The character is very small, and any function key will work as a character. You can only type the function key once, then it will not let you continue to type more. When you type any other characters in front of it, it disappears.
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Can't reproduce
Please force a crash by pressing F3 + C for 10 seconds while in-game and attach the crash report (minecraft/crash-reports/crash-<DATE>-client.txt) to this ticket.
Can you elaborate on 'function key'? Do you mean a modifier key like ctrl, shift or alt?