Hi Lucas,
If that is some sort of a custom posts widget (not the built-in Elementor Posts), then it may not work. The live filtering feature is made for the elementor Posts widget – 3rd party widgets may work completely differently, and there is no guarantee they will react to the plugin query.
The question I answered was about supporting custom post types, which were created by a 3rd party plugin. This is something completely different – this is an elementor custom posts query widget, and it probably will not work.