As of snapshot 25w43a, Minecraft now runs on Wayland natively instead of XWayland on Linux. This comes with unfortunate side effects.
While the control key is held, typing letters is supposed to only have the special effects (e.g. ctrl-c to copy, ctrl-a to select all). But currently, the special effect happens, and the normal letter is typed. So ctrl-a selects all, then immediately replaces everything with a single letter A.
Steps to reproduce the issue
Make sure you are using Wayland and not X11
Launch snapshot 25w43a
Open chat
Type something
Press CTRL-A
Expected result
All text should be selected
Actual result
Everything is replaced with the letter A
Environment
Arch Linux 6.17.4-arch2-1
KDE Plasma 6.4.5
Wayland
Can confirm that this also happens on Fedora 42 running Gnome 48.