HR goes DOWN when I exercise?

I've just got a VS4, replacing a Polar FT7 after about a decade. Having been using a HR monitor for a while, I have a pretty decent idea of the sort of ballpark figure I should be getting for various levels of activity.

I just(as in, half an hour ago) spent an hour working in the garden, at a level I would expect to be in the 130-140 range, but according to my shiny new exercise monitor, my average heart rate was 102. Then as I was getting back to normal, it jumped up to 112. 

I previously returned a device to withings for being unable to read a pulse in the presence of sweat(which, you know... exercise? You're supposed to sweat.), so I really hope this isn't a universal issue. It would be pretty bloody stupid for the standard exercise monitoring tech to universally work just fine when you're resting, and stop working when you start exercising BECAUSE
you've started exercising.

This is more than a mild annoyance because I am relying on this device to report back to Vitality to lower my premium, so if it refuses to work accurately, it's actually costing me money.

  • I also just purchased a VS4 yesterday.  Used it at the gym for the first time today for some strength training.  The heart rate sensor/monitor didn't want to go above 105bpm, where on my Polar H10 chest heart rate monitor my BPM were well in the 160's at times.