You actually kinda can reproduce this outside of gnome-shell password prompts. If you manage to exit a gnome-shell password prompt while a standard GTK textfield already is focused, and then you try to directly type uppercase text, it will ignore the shift key press.
You can easily reproduce that with the Seahorse dialog I took a screenshot of, but I also managed to reproduce it with a password prompt in Nautilus.
You actually kinda can reproduce this outside of gnome-shell password prompts. If you manage to exit a gnome-shell password prompt while a standard GTK textfield already is focused, and then you try to directly type uppercase text, it will ignore the shift key press.
You can easily reproduce that with the Seahorse dialog I took a screenshot of, but I also managed to reproduce it with a password prompt in Nautilus.