OOTB Expected Revenue Becomes Zero After Opportunity Creation or Update

We are using the OOTB Expected Revenue field on the Opportunity. The field is enabled in the UI, and we enter a value during Opportunity creation. However, after saving the record, the Expected Revenue value becomes zero when we open the Opportunity in the edit page. The same behaviour occurs even when we try updating the value directly from the edit page—after saving, it resets to zero again. There is custom logic impacting this field. We need to understand why the value is not being retained, what internal logic is forcing it to zero, and how this can be corrected.