watchOS4 loosing bluetooth heart rate chest straps

Hi, I've always used a bluetooth heart rate chest strap with my "Series Zero" watch while exercising (more consistent reading, irrespective of activity), and was always a matter of just pairing and connecting the first time, after which, whenever the watch saw the heart rate monitor, it would just connect.

However, since updating to watchOS4, not only does the watch disconnect from the chest strap, but will even unpair or totally remove it, taking anywhere upto 30 seconds just to see the chest strap as a bluetooth device.

I have two different makes of chest strap (Polar and Wahoo), both behaving the same way on the watch.

Anyone else seen this or have a fix?