In CRM 2016 on premise environment on an opportunity product form the default Value for attribute Select Product (isproductoverridden) is not respecting the customization. The default value is set to "Existing", but when you create a new Opportunity Product the field Select Product is set to Write-in.
There is javascript code that executes on the form load. It checks to see if this is a new Opportunity Product record. If it is, it defaults the value to Write-In.
This issue is by-design even in CRM 2015 version.

This is major concern from a user experience stand point because even if a user is intending to create an 'Existing' type opportunity product it gets defaulted to 'Write-in'.