Repro Steps:

1) Create a new Project with a MainPage.xaml
2) Introduce a ComboBox to the Page, right-click on it, navigate to the 'Styles' context menu, and create a new Style for the ItemContainer.
3) In the Properties Panel, hover over any of the Properties with an orange outline around them ('Background' for example), and click on the 3-Dot Menu.
Observe how there is a [built-in] tag at the top of the context menu.
4) From the context menu, select 'Remove Setter'.
Observe how there is no result produced.

5) Using the colour picker, select a new background colour for the Background Property.
3) Hover over the Background's 3-Dot Menu, and select 'Remove Setter'
Observe how the Setter is successfully removed.

This appears to be an issue with Noesis built-in Styles, which serves the 'default' values for this Element's Style.
Therefore, there should likely be a check that if a Property in a style comes from a built-in resource, that the 'Remove Setter' action should be disabled.

(It also appears that the "Go to Resource' arrow in the Property's 3-Dot context menu also doesn't perform a function whilst a Property is using a default/built-in Resource)