Apple Health export file is corrupt(?): cannot be unzipped

I'm trying to back up my health data from an iPhone 15 Pro running iOS 26.0.

I go into "Health", click on my icon in the top right, scroll down to "Export All Health Data", then wait about 10 minutes. At this stage, I've tried transferring the export to my desktop Mac via both AirDrop and by saving to my iCloud Drive, with the same results.

The result is that I get a file called "export.zip" on my desktop Mac. I would have thought that I could unzip this file to yield underlying XML and GPS tracks, but trying to unzip this file (by double-clicking on it in Finder) results in an error:

Unable to expand "export.zip" into "foldername".

(Error 94 - Bad message.)

(with the string foldername replaced with the name of the folder in which I have placed export.zip)

Can anyone tell me what I'm doing wrong? Or is Apple Health data write-only?

Thanks!

-Mike