The widget appears in a wrong location for an instant

If a predefined or custom location has been configured from the Widget location module but the Products Reviews by Shopify snippet is still present in your theme templates, then you may see how the widget appears for an instant in the snippet position, and then it changes to the definite position, causing a bit awkward effect. 

If it happens, simply remove the Products Reviews by Shopify snippet in your theme templates and Product Reviews Add-Ons will manage the definite widget position with no awkward effect.