photos suddenly won't download from icloud as jpg files

For as long as I have used icloud, I have been able to click on a photo and use the "download" icon at the upper right corner of the page (next to the trash can) to open a photo in Windows Photo Gallery as a jpg file. I could then save it to my PC.

Suddenly, out of nowhere, the download icon now creates a zip file containing an HEIC file image, which Windows Photo Gallery cannot even open. This includes photos I took years ago, and previously have been able to download/open as jpg files.

Why would this suddenly change? (I did not change any settings on anything.) And how can I change it back?