This problem was pointed out on the German-language Wikipedia's feedback page yesterday. It might be related to the second problem in T145268: Always show and update the category display at the end of the page when editing in VE.

When you

1. go to a category page, e.g. Kategorie:Hunde
2. click edit to enter the visual editing mode

you see the content of the category at the bottom:

However, when you

1. go to a category page, e.g. Kategorie:Hunde
2. click edit source to enter the source code editing mode
3. click on the pen in the upper right-hand corner of the editor to switch to visual editing

there is no category content shown:

One would expect to see the same listing of pages as when directly opening the page in the VisualEditor.

If one first opens the page in visual editing mode, then switches to source code editing, and then switches back, no category content is shown either.