HealthKit "backfill" doesn't write to HealthKit for previous days.
It appears to accurately import data FROM HealthKit for many days in the past, but it doesn't update data TO HealthKit.
This is relevant when using the "update past entries" for custom foods, meals, recipes — for example, if I realize I've been undercounting water in a protein shake or something, I'd like to update previous entries in Cronometer, but I would also like HealthKit entries to be updated as well.
The only work around I have found is very tedious — going back to each day and adjusting one food (even just a minute on the timestamp) — this causes the HealthKit data to update.