You have created a child opportunity, however, it does not appear in the parent opportunity's document flow tab.
SAP Cloud for Customer
The document flow tracks an object from where it originates to a quote and order. A child opportunity is not a preceding or follow-up document, so it will not reflect in document flow.
This is the expected behaviour of the system.
opportunity, child opportunity, document flow , KBA , LOD-CRM-OPP , Opportunity Management , Problem