Last couple of years Apple made a very big deal in their product announcements that ResearchKit, along with iOS embedded features would help healthcare pros and patients conduct "ground breaking research" using their devices. Yet I just looked at my steps, and I see 3 of the 7 days completely missing data.
If I was in the field, I would have a hard time trusting Apple with my research data going through ResearchKit when they can't do something simple like tally up and record a number of steps. Because ultimately, this type of data may make it to the FDA, and the FDA won't take "our researchers were missing data from iPhones and therefore this drug passed" as an excuse.
http://www.apple.com/researchkit/
Your missing step data because of this bug. This bug prevented new data from being written along with accessing previous data. This also did not impact everyone. Research kit is also completely different than tracking steps.. Those are actual apps that are using ResearchKit APIs. Those apps would continue to function even if health data was not being recorded. This is a bug in the health app only.
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It didn't work in beta 2 and it was closed. Either way, let's hope they fix it.
I ran all of iOS 10 betas. It first appeared in 10B6. This issue popped up after they fixed the scrolling to the bottom of page bug in Safari. That was a keyboard issue, so clearly some code got crossed.