After updating the plugin, did you clear Safari’s browser cache?
Unfortunately I do not have access to a Mac right now, so I can’t test MacOS Safari.
But I did try it on an iPhone and a PDF did preview in iOS Safari, although the preview sometimes did not load right away until I refreshed the page.
If clearing Safari’s cache doesn’t help, try the SHIFT + O keyboard combination to view the same ticket in vanilla osTicket. Does the PDF preview display as expected over there?
(BTW I highly recommend using Chrome on MacOS anyway. I think it is a crime that Apple keep promoting Safari. Either they should accelerate its development, or they should abandon it. But that’s my personal opinion, ymmv)