I got the iPhone 6 the day it was released. I immediately started using the new Health app. I added the basics, birthday, age, sex, weight, and I added the "Steps" and "Walking + Running Distance" to the dashbaord. I notice they automatically started tracking my steps. So I went for a jog, it tracked that also. Awesome, so far so good. But the next day it was all gone. It doesn't matter if I select "Day / Week / Month / Year" at the very top, the stats are always the same, they only show the current day. I've checked each day since I've got the phone, each day it seems to start over. Why is all the previous data disappearing?
This is a total speculation but I think this might be because the data you are viewing is generated by the M8 co-processor and it only stores data for a relatively short space of time. The idea being that other apps are meant to take the data from the co-processor and store it, and the M7/M8 co-processors themselves just have a little memory to cover periods when the device is turned off.
My guess is that if you install another app that tracks movement (once you have installed 8.0.1, when it is safe to install), then they'll take over the role of storing and supplying data to the Health app.