It seems others have a similar issues, but I'm posting here to give it further visibility and to show to very clear examples.
I've never had an issue with the heart rate sensor before, and in fact I've always been very impressed with its response.
But after the most recent update heart rate tracking is very unreliable during the first 10 minutes of an activity. Below are two examples: the first was a daylight trail run, and the second was a dark road run. I wore the watch in the exact same way as normal both occasions.
There is a big 40bpm jump in heart rate after about 8 minutes or so. The runs were steady - so this is just tracking error. The higher heart rate is correct, and what is usually recorded.
Just to emphasise, this has only occured since the most recent update so I am very inclined to believe it is software related. A fix would be most welcome - it is currently rendering many of the Garmin statistics useless.