Hook path in main menu not hold active trail
I am not sure what you are doing in a lot more areas of your system than described above, but doing the following on a d7 testbed here:

function MYMODULE_menu() {

  array $items;

  $items['about'] = array(
    'title' =&gt; 'About',
    'page callback' =&gt; 'MYMODULE_about_page',
    'type' =&gt; MENU_NORMAL_ITEM,
    'access callback' =&gt; TRUE,
    'menu_name' =&gt; 'main-menu',
  );

  $items['supply-area'] = array(
    'title' =&gt; 'Supply area',
    'page callback' =&gt; 'MYMODULE_supply_area_page',
    'type' =&gt; MENU_NORMAL_ITEM,
    'access callback' =&gt; TRUE,
    'menu_name' =&gt; 'main-menu',
  );

  return $items;

}

function MYMODULE_about_page() {
  return 'What\'s it all about, Alfie?';
}

function MYMODULE_supply_area_page() {
  return 'Making Supplies';
}


...followed by clearing caches and then setting it up via the Menu UI:



...provides the following:



so I would check how you might be doing it slightly differently on your system.