Health Data Export gives me empty zip file

When iOS 11 came out, it had a nasty change/bug in the way it handled health data. Our family's Apple gadgets have always been under the same Apple ID (please no comments on how that's not allowed - it certainly was), but with the upgrade, I started seeing my wife's health data on my iPhone (and vice versa!) On its own, Apple decided to put my wife's phone & watch a "data source" for the Health up on my iPhone (and vice versa). In the process it even created a few duplicate data sources. Anyway, it was a mess - our data was pretty intermingled. I filed a case and Apple's acknowledged the issue - but never gave advice on how to undo the damage (the case is still open AFAIK).

Anyway, since we were going to get iPhone X anyway, I decided to start out with a fresh slate. I removed both old iPhones from iCloud so that when we began using the new IPX, it wouldn't download all that corrupt data. Both our phones work well now and have begun getting data form our watches (and our smart scale) without pollution from the other. Now I want to start the process of importing the data from the old phone. First step is to export it. I go into the Health app, tap on the user figure in the top right, and tap on "Export Health Data". After warning me that this might take awhile, it eventually (we're talking 10-20 minutes!) comes back with a popup that lets me either email it or put it in Files or...I chose "email" since the other mechanisms seem to require me to be connected to iCloud or otherwise identify myself with Apple ID (which I don't want to do for fear of mingling data again).

It appears to work - at least I get an email with a zip file. Unfortunately, the zip file is empty :-( Has anyone experienced this? Does e-mail only work if sending to an iCloud.com email address? I have no idea what's going on. I know I have health data - I see hundreds of data points for body measurements and activities in the Health app.

I tried to go the route of using AirDrop or writing to Files (iCloud) - but neither seems to work (and each time I have to wait 10-20 minutes while it "prepares" the zip file).

Any help much appreciated.

(I haven't even gotten to my next question: if Apple provides an "Export", where is the "Import"??? I haven't seen such an option).