On the Watch: watchOS 4.3 beta 3
On the paired iPhone: iOS 11.3 beta 4
Observed behavior: Activity numbers (Move/Exercise/Stand) stop updating on the Watch, until the Watch is put to Airplane mode/restarted. If nothing is done - data is lost completely. In particular, Stand hours are missing and marked as "empty", like the watch was not even on the wrist during that hour. Was previously observed with watchOS 4.3 beta 2 and 1 (and iOS 11.3 beta 2 and 1), but was fine with iOS 11.3 beta 3. After iOS 11.3 was updated to beta 4 - problem occured again and stay there.
Remarks: with this iPhone two Apple Watches are paired. One with watchOS 4.2.3, another one - with watchOS 4.3 beta 3. However, the Watch with watchOS 4.2.3 is completely turned off for a week already.