[Plugin: subpages-widget] shows full top-level menu on subpages

I installed and configured the subpages-widget plug-in, and it works fine as a sidebar widget on top-level pages, but on sub-pages it displays the entire top-level menu instead of only the parent and sibling pages.

In other words, it displays:
-on top-level pages with children: current page as title and subpages as links (correct)
-on top-level pages with no children: nothing displayed (correct)
-on subpages: all top-level pages (and all subpages if that option is configured)

It’s supposed to show the same menu on subpages as the menu shown on their parent page.

Any suggestions how to troubleshoot this issue? Is it a problem with the theme?

Thank you.