Custom tab not visible when accessed from custom menu

HI, 

I have created a custom tab ‘Register Arrivals’ in Purchase Order window.

I have also created a custom menu Purchase Order in ‘External Supplier Invoices' window.

Scenario 1: If I access directly Purchase Order window ==>  Custom Tab ‘Register Arrival’ is visible.

Scenario 2 : When I open ‘External Supplier Invoices' window/ RMB ‘Purchase Order’  ==>  Custom Tab ‘Register Arrival’ not appearing in Purchase Order window.

I noticed that Register Arrival tab will be visible again in Scenario 2  if I remove the param VERSION=SINGLE_COMPANY from the URL . 

Any thoughts would be much appreciated.

Thanks