A couple of reasons this might not work.

1. On the 3 step manufacturing, Post production to stock move is achieved with a push rule. The products in this location would always be reserved by the operation, hence will never be available for picking to Output

2. The replenishment (MTO) is likely configured for Stock location and not at the post production location.

Raw material room is probably a child of Stock location enabling replenishment to trigger.

If you wish to always pick from Post production the suggestion would be

a. Set to 2 step manufacturing

b. Add a manufacturing route to Post production location.

ie when product is required in customer location, pick from Post production and push to output; if unavailable trigger a manufacturing operation