It seems the FlexSlider only displays on Portfolio Items ONLY when UPLOADING images to a gallery to be inserted into the Portfolio.
FlexSlider does not display if you insert a Gallery made by selecting images from the media library.
I presume this has to do with the quirky ‘Attachments’ method employed by wordpress. Images uploaded to a post become attached to it. Just adding images from the media library does not create the attachment.
While racking my brain on why a Portfolio Item refused to show the FlexSlider, I found this post: https://theme.co/apex/forum/t/how-to-get-flexslider-in-portfolio-item-page/13853
Please re-work FlexSlider to use a Gallery inserted or edited in a given post. And NOT be based on attached images.
For others finding this issue - there are plugins out there that help manage Attached files.