There is a long standing bug in the menu system in Gantry 5 and wordpress. The leads to the wordpress menu getting out of sync with the menu in Gantry 5 menu editor. To get around this you should do all your menu changes in WP and then...
1. go to /wp-content/themes/g5_helium/custom/config/menu and delete the YAML files located there.
2. Go to Gantry 5 admin menu editor and just press save on each of the menus - this will recreate the YAML files.
IMPORTANT CAVEAT: if you have done any changes in Gantry 5 menu editor (e.g. columns, added particle or widget, or made other setting changes - THEY WILL BE LOST and you will have to reapply them.
Just to be clear as well (I'm not sure if this applies to your case) you can only have "columns" at the first level of dropdown. You cannot added columns at lower levels of dropdown.
As always, make a backup before you start these changes.
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