The poll function of the "Reset to Default Value" operator fails in the popup menu.
This is because it cannot find the active uiBut.
This is because CTX_wm_region(C) does not return the region of the currently open menu popup in ui_context_rna_button_active.

A quick hack to make this particular case work is to replace the function with:

static uiBut *ui_context_rna_button_active(const bContext *C)
{
  bScreen *screen = CTX_wm_screen(C);
  if (screen && screen->regionbase.first) {
    return ui_context_button_active(screen->regionbase.first, ui_context_rna_button_active_test);
  }
  return ui_context_button_active(CTX_wm_region(C), ui_context_rna_button_active_test);
}

This breaks other cases though. I assume the real fix would have to make sure that CTX_wm_region(C) actually returns the popup region, don't know for sure.

The preferable way to achieve this is using CTX_wm_menu(), it returns the menu region if there is one. If it returns NULL, the current CTX_wm_region() can be used.

However I don't know if this change is safe to do, after all this is called from many different places.