the windows blog says it fixed the band sync issue with 10051 but maybe that didn't include connectivity issue?
The bug they're talking about there is that in the last build, it couldn't sync with the band at all in most cases. That build, you open up the health app, tap sync, and you get an error. New build, you open up the health app, tap sync, the band syncs. So that's correct, that fix doesn't have anything to do with continuous connectivity.
So, yes it is a bug that can't be fixed on the user end AFAIK. The band won't stay connected on the current build of W10TP. My guess is it's from some sort of Bluetooth power saving thing in the drivers that drops the connection until the phone needs it, not knowing that the band at some point might want to talk to the phone for something like Cortana. For example, my band stays disconnected from my L830 all day until I get an email, at which point my phone connects, sends the email notification across, and immediately closes the connection.
(However, I annoyingly don't get text notifications, but that might be another issue entirely with the way W10 currently handles notification forwarding under the hood... Remember back in earlier versions of WP8 where the use of smartwatches for phone notifications just wasn't possible because the only app that could give text notifications was the native texting app?)