If one presses in the text line in Minecraft, A disappears everything?

So, my brother is currently playing Minecraft and just wanted to use this chat function to write to his friend but every time he presses the A key, everything he has written disappears, what can one do there.

Ps: Restart game works only briefly. And after 10-15 minutes it is the same again.

Maybe he presses Ctrl + A, so that you mark everything in the chat and the next button press disappears accordingly everything because it is replaced with the pressed letter.

No, I also checked it more times because I thought so too