So in the Health app, Activity - when you tap on the Fitness icon (the one with the rings) it pops up a calendar - look at the dates in question and see if it is the same as the activity app. Interesting. I never knew about the calendar in Health. Anyway, when I select dates prior to August 2023, it shows no data. I also notice that the calendar only goes back to Aug 2021, and that makes me wonder what happened to the data dating back to 2015.
on the phone - Settings, General. iPhone storage, Health - at the bottom Data Sources, takes maybe a minute to display, but does it show data for all the 10 AWs that are listed? Yes, it does.
Do you recall anything related to your AW or phone that happened in that Aug 2023 timeframe? like an un-pair/re-pair, a new watch, an new phone or anything like that? It's possible there was an un-pair/re-pair, but I don't recall. Every watch and iphone I've bought have been in September (at introduction).
how did you delete your health data if you recall? I did it following the instructions of an Apple support person. As I recall, it was: Settings>Apps>Health>Data Access & Devices>Health>Delete All Data from "Health" Her thought was that deleting it from the phone would force it to reload from iCloud and thereby retrieve the missing data. Good idea, but it didn't work.
As you know health data is backed up/sync’d with iCloud and my interpretation of that relationship is that deleting data on your phone will delete the data in iCloud when you deliberately delete on the phone, not if you erase and setup a phone. I think that might have happened to you, I could be wrong of course and it is odd that you did this in March or later that you still have data back from Aug 23. I wondered about that myself. The Apple support person guaranteed my that deleting it from the phone would not delete it from iCloud. Before I deleted the data from the iPhone, the oldest data was August 2023. After you delete the data, iCloud slowly puts it back on your phone. A day later, all the data was back, but again, only back to the exact same date in August 2023.
What is really odd, it that there are random dates in the past that show data. For example, September 20-24, 2021 shows activity. As an experiment, I have tried adding move data to a 2023 data that didn't have any Health>Active Energy>Add Data. When I do that, the added calories show in the Active Energy tab, but still don't show in the Activity tab.