Not sure if this was resolved but from what I’ve read thus far, it doesn’t appear to be - however, I may have solved it for my case and it may work for you as well - lemme know.
I was getting this problem too - and yes, I had a clipping mask applied to one of the viewports as well so it definitely the culprit - this was confirmed as I deleted the viewport in question and it exported to PDF as expected.
So, I undeleted the viewport and I switched the Page view to another page other than the one I wanted to export (not sure if all of you have that exact page on your screen which you’re trying to export to PDF but I did); anyway, as soon as I had another Page that was displayed on the screen other than that Page that I was trying to export, odd shaped-clipping masked viewport included, it export to PDF just fine!
Lemme know if this works for you.
kree8 Design Inc.