Confirming. See it here

That's very odd. The sorting is using standard function gtk_tree_sortable_set_sort_column_id. It worked before. Not any more ...

That code was not changed since 2.32. It looks like some regression in gtk+

After some testing:
If I call gtk_tree_sortable_set_sort_column_id before actually populating liststore - it works. If I do that when liststore is already populated - it does not work.

Created attachment 178575 [details]
testcase

Here is a small testcase. Setting the sort column before adding the model to the combo box works, doing it afterwards does not.
Treemenu regression, I'd say.