The manual for the Fenix 6 says that the heart symbol on the watch will flash while it is getting your heart rate, then go solid when it gets it. In reality the heart icon is solid black when not on my wrist; it never flashes to indicate it is searching for heart rate then goes to pale grey when it finds my heart rate.
I think it is actually working and recording my heart rate but it would be good to know that it is actually working correctly (and therefore the manual is wrong) because it is totally different from the way that my Garmin 235 works - that does flash then go solid as it is searching for, then locks onto heart rate.