Unfortunately, I couldn't find this information anywhere at the manufacturer's or on the web in general. With a bit of luck, someone else will answer your question.
I regularly do Cardin training with the Forerunner 35 (usually twice a week), and I did not notice that the pulse would not be recorded permanently. Sorry, I cannot confirm your observation.
Ciao
Wrist pulse measurement works by measuring the blood flow on your wrist.
If the watch is too loose or there is too much sweat under the watch, your watch will not be able to measure correctly or the measurement will be disturbed by various factors.
Therefore, it can happen that the pulse measurement does not match your heart exactly to the beat.
If you want to have a really accurate heart rate measurement, it is best to try the Garmin chest strap. The chest straps are much more accurate.
Greez
whitelemur