Please demonstrate on apex.oracle.com with providing workspace name and a developer account
I already provided an example page, did you see that? The PCT column source is trunc(dbms_random.value(0,100)) with default column settings.
The Show Value attribute has no effect at all. It was never implemented. This is not specific to Interactive Grid also happens if used as a stand alone page item. The functionality to show the value was not in interactive report.
This is tracked with bug 27978977.