Bill/Rev Plan created with Ext Source & Ext Key via Contract API not reflecting in Contract Line UI

Summary:

Create Contract with Project API - /fscmRestApi/resources/11.13.18.05/contracts

Bill Plan and Revenue Plan created via Contract API (with Ext Source/Ext Key) are recorded successfully in the response, but are not reflecting as linked to the contract line in the UI. Seeking feasibility confirmation on whether existing plans can be directly linked to the line, or if a second PATCH is mandatorily required.

Content (please ensure you mask any confidential information):

We are creating a Contract through the REST API, where the contract line includes a Bill Plan and Revenue Plan, each passed with an Ext System Source