Alt for 3rd Level no longer works in browsers

It works everywhere except browsers (works in Falkon).
It used to work before, but now if I hold the right alt key in a browser, I can’t type any letters.
Using a key other than alt works fine.

% uname -a
Linux HOSTNAME 6.3.8-arch1-1 #1 SMP PREEMPT_DYNAMIC Wed, 14 Jun 2023 20:10:31 +0000 x86_64 GNU/Linux

% pacman -Q plasma-desktop                                                                       
plasma-desktop 5.27.5-1