Rails Engine path mismatch in Rails 3/4

Our rails app has quite a few rails engines and is deployed in production under sub_uri /my_uri. Here is how the orders_path index page is referred in engine named po_engine:

po_engine.orders_path

po_engine is mounted on '/po' in routes.rb.

In rails 3.2, po_engine.orders_path returns url:

/po/orders

In rails 4.2, the same po_engine.orders_path adds the my_uri and returns:

/my_uri/po/orders

The configs in environment/production.rb are the same as we understand. This change creates mess in our views on Rails 4.2. Besides making changes in views, can someone point to a config change in such as way Rails engine path work the same way in both rails 3 & 4?