Enduro 3 heart rate issue

During the past two weeks my heart rate reading seems to get stuck between 115-125 BPM when I know it’s up around 165-170. Sometimes I have to end the activity and restart and sometimes I have to turn the watch off and back on for it to work. When it does fix itself it instantly jumps to the correct heart rate. Any idea how to fix this? Was it caused by an update? I’ve had the watch for about 6mo and it only just started happening.