Saving As Optimized PDF to "flatten" fillable ("live") fields no longer works

For years now, I have been creating Adobe Acrobat documents such as Invoices with fillable form fields, allowing me to use the same basic document over and over again to create a different (new) Invoice. To get the final version that will go to the client (with no fillable fields) I would "Save" the document, then "Save As Other" and select "Optimized PDF." However, this process no longer works. No matter how I save the document, the fillable fields are still there. How can I get this resolved? The ability to "flatten" live form fields is the main reason I have been paying for the full version of Acrobat.