Forerunner 255 series HR Data delay during run

When I start a run, my heart rate increases and then hits a plato at just below 120 bpm. After 10 minutes, it suddenly increases to 160 bpm, where it should be.

I am on the latest sofware version, but the problem keeps persisting. I have started a "fake run" just for the watch to get going, with the hope of getting the correct HR data with an actual run, but no positive result. This method has worked in the past