This is more of a design flaw than a bug, but an issue none the less. It appears that in 1.19-pre3 the pause key is the physical escape key rather than what X11 generates as escape. In particular, I am running Ubuntu 20.04.4 LTS and (as a vim user) have swapped caps lock and escape. I have tried switching them using Xmodmap (through gnome tools) and by editing the us keymap in xkb/symbols. (There may be a more fundamental way to switch caps lock and escape through the locale that is independent of X but I am unaware of it)

Minecraft 1.18 uses X11's escape key (and thus Minecraft respects the switch) but 1.19 pre3 seems to use the physical key (and thus does not respect the switch).

Some solutions to this might be: return to how it was in 1.18 (read from X/OS) or add a keybinding for an extra pause button (so that pause can't be lost - presumably this is what the change was about).