Resolution:
This is an expected behavior where we do not show a total aggregation for amount fields that are not within the object being used by the selected mode. To explain this, let’s take the example of the above screenshot.
If we check the field Deal size, it doesn’t have the Total aggregation whereas the Product Total Price field has the total aggregation.
If we check the API name of the fields, the Deal size field’s API name is opportunity.amount and API name of Product total price is opportunitylineitem.totalprice which means both the fields are coming from two different objects, one from Opportunity and other from Opportunitylineitem and as in this case, as we are viewing the data in Products mode i.e. opportunitylineitem object, the amount fields from this object will have Total aggregation whereas any amount field from any other object will not show the Total aggregation.