I came back from a run yesterday and noticed that my activity did not automatically sync to my phone. After digging around in the Connect app, it looked like my watch was completely disconnected from my phone. After several attempts to re-pair, I ended up resetting my 255 completely, uninstalling/reinstalling the app, and clearing my Bluetooth cache, all with no success.
I'm running the latest software on my watch, version 19.18. I also don't think it's a problem with my phone or the app because I tried pairing with an old 45 I have lying around and that worked just fine.