Trouble with Adobe PDF Color Profiles / Saving Results

Hi all, 

For years until very recently, our agency's go-to method for sharing for-print proofs for on-screen viewing has been to work within a CMYK document setup, but save/export as an Adobe PDF with the color output converted to destination -> Working RGB profile. 

This generally resulted in screen-accurate colors to those that would eventually be printed. Generally, all of our colors are assigned from a Pantone bridge palette. The resulting PDF could be compared over the working file on the same screen, and colors would be very accurate between the two. 

Begining some time very recently, this is no longer the case. We're seeing very different resulting colors from the saved PDF vs the working file in Illustrator. There seems to be no difference between leaving the color profile as CMYK, and of course CMYK colors viewed in an RGB/Web browser are never accurate.

Going through my normal export method:
CMYK Color Mode Document: Save As -> Adobe PDF -> (General) Optimize for Fast Web View ->(Output) Convert to Destination-> Working RGB-> Include Destination Profiles

Attached screenshots are working file on the left, PDF on the right. The vibrance, and especially the blue tones, are very different, especially noticable with larger coverage areas. 

I would really appreciate any and all insight on what may have changed, and how best to adjust so that PDFs save with on-screen colors similar to the working file. 

Thanks!