I’ve actually had the exact same issue recently.
A customer left a comment on one of my best selling watch faces (made with newer version of GWD, after 1.6.2) that all health data displays “0” for them. It’s a watch face with lots of health data, and this was the only time someone reported this issue. All other comments were highly positive, so I’m 100% sure it’s not caused by the watch face itself.
I’ve suggested the buyer that he/she needs to allow all permissions when installing for the data do become visible, but since the comment section is limited (only one reply and potential edits), I’ve only managed to find out that the user claims to have done “everything they can” and that it still displays “0”.
I’ve suggested the buyer to contact me through email with more info so I could find out more and hopefully help them, but they haven’t contacted me back.
I’m guessing they haven’t tapped the little “check” icon after the installation to allow all permissions, or they might have missed it, because I have no idea what else could have caused this.