Salesforce has differing behaviour between Classic and Lightning interfaces.
If the Opportunity Settings checkbox to ‘Prompt users to add products to opportunities’ is checked and the user is working in Classic and the Flexpricer button overrides are in place, then Flexpricer’s Vision Product Selector automatically appears when an opportunity is created.
However, if the user is working in Lightning, the Salesforce Product Selector appears instead.
Whilst inconsistent, this behaviour is controlled directly by salesforce.
In the situation, we recommend unchecking the checkbox to Prompt users to add products to opportunities from Setup | Opportunity Settings and loading the Product Selector after the opportunity has been created.