Input source indicator not shown

In my 23.04 development installation I installed the ubuntu-unity-desktop package and then logged in to Unity. One observation is that the input source indicator is not shown in the top bar, even if that feature is enabled in Text Entry. Please see the attached screenshot.

I can successfully switch between the input sources in the list using Super+Space.

First I suspected that this ibus patch has something to do with it:

https://salsa.debian.org/debian/ibus/-/blob/master/debian/patches/ubuntu-forward-panel-properties.patch

It's there for Unity only. But since the issue is present also with ibus disabled, that does not appear to be the case.