Drop Shadow fails to export to png

A strange problem cropped up for me yesterday - all of my Drop Shadows failed to appear on a png that I exported. I'm a reasonably experienced user and have done this plenty of times in the past without a problem. The shadows are definitely there on screen, but just not in the exported png.

I'm using Illustrator CC 2015.3 on a Win10 Home machine, 16GB of RAM. The only new element in my setup is a new GPU card - nVidia GeForce GTX 1050Ti, with 4GB of RAM, which I've been using for a week or so without problems, but I've never put through this exercise before.

For what it's worth, I did some benchmark speed tests before and after installing the new card. Saving my (huge, over 400Mb) Ai files didn't really speed up much over my old card's time - still took around 5 minutes. Printing to a PDF file still took about 30 min. But exporting to png went dramatically faster, going from like 30min to something like 3min. I figured, hey, that's just the nice thing about having a modern GPU as opposed to my old OEM card. But now I'm wondering if it went so quickly because it wasn't rendering the drop shadows. (I didn't check at the time).

Any advice? I'm hoping the answer isn't "update Illustrator to the latest version" because I don't trust it to work. Having seen all the problems people reported with Illustrator 2020 ("problems"? More like "catastrophic failures impacting people's livelihoods"), I'd rather just stay warm and safe with my old version that works. (If it works).

Thanks,

Tom