We get numerous feedback from Windows insiders that there are problems with synchronization on build 14379 (and probably 14376). Unfortunately, we can't do anything on our side. Seems like some Bluetooth low-level caching problem.
A temporary solution to the problem would be a restart of your phone before synchronization as some people report. Otherwise, you need to wait for the next update of Windows.
We can only say that the news is good and bad at the same time. From one side, we don't recommend to use unstable builds, but from the other side, it means that Windows team is doing at least something and (hopefully) is moving forward.
UPDATE: as it is reported by users, in build 14383 everything goes well with sync.
Regarding Mi Band 2: it was really hard to get it and I ordered it from 2 stores. After weeks of waiting, now both of them are shipping. I expect to get first of them next week and start exploring (I don't know what I'm going to do with two bands, but this is another question...)
UPDATE: I received Mi Band 2 and started to implement support of it. It will take some time, since protocol has changed greatly.
See you soon!