HRM-Run: inaccurately recording HR.

I invested in the HRM-Run about a month ago because my Forerunner 935 was occasionally recording my HR near the end of a run. After a month with no issues, today the HRM-Run shows my HR jumping over 40 beats/minute instantaneously, and then remaining at that high level (some 20 beats/minute above my max HR and 35 beats/min above any prior recording, despite that I was maintaining a steady and not uncomfortable pace on a treadmill with zero incline. Any advice for correcting/avoiding this in the future? See attached photo.