Issue creating relationship with custom module via API

I am using Bitnami’s Docker image for version 7.13.2.

I’ve created a custom module Customers from Module Builder with a one-to-many relationship with Contracts with Customers as the primary module. When creating the relationship between an existing customer and contract, the response appears to be successful with a 201 status code but the relation was not actually created.

I have tried “Rebuild Relationships” and “Quick Repair and Rebuild”.