Since the 12.10 update my heart rate has been fairly inaccurate, especially when pushing myself. With the 12.10 update I found it went both high and low, like 110 pushing hard up a long uphill trail run, but would then race up when coasting down. I've tried cleaning sensor, swapping wrists, reset...
The 12.52 seems to be even worse, I now only get low and medium rates, anything above 160 and it'll crash back down to 110, or even sub 100. I'm now trying the beta, but they seem to be the same version.
Not that I care too much, but my VO2 max is going to start dropping as I'm never getting past threshold, but if like to have accurate HR data, but not have to wear a HR strap