Keyboard layout selector is missing from the indicator applet in ubuntu mate

I'm running Ubuntu Mate 20.04.03. At some point in the past, it was possible to select between multiple keyboard layouts using a drop-down in the indicator-applet in the panel. This is no longer the case. Multiple keyboard layouts are enabled on the system, but the switching option is missing. I'd like to get this feature back, especially because I remember it including an on-screen (mouse) keyboard which is handy.

> cat /etc/lsb-release 
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=20.04
DISTRIB_CODENAME=focal
DISTRIB_DESCRIPTION="Ubuntu 20.04.3 LTS"

This might be a duplicate of [this question](https://unix.stackexchange.com/questions/102943/keyboard-layout-panel-applet-in-mate-1-6) , but there's no way to tell whether that issue was due to the same bug. Also, that question has no valid answers, so we'll need to sort this out anew regardless. I also [can't type backslash anymore](https://askubuntu.com/questions/1376518/i-can-no-longer-type-backslash-in-ubuntu-on-a-lenovo-p50) but I think that's an unrelated issue.