iOS 8 Health app stats resetting each day
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.