Just got my friend's Fenix 5, and have been noticing very different HR readings, compared to my Polar S720i.
i have on both the coded Wearlink (not bluetooth), and the Garmin straps. both watches will give the same readings at rest, but when i engage in activity (ie: cycling), the Fenix will read 10-25 bpm higher than the Polar, and will fluctuate, and get "stuck" on a number, while the Polar maintains consistent readings.
the battery is good, the strap is fitted snugly, and the battery contacts are tight (which is an issue i have with the polar pod).
i'm pretty sure there is no cross-talk, because i've tried every combination of belt/trasmitter pod and the Garmin belt/pod is the only combo that works, and i"ve also went out on the bike with just the Garmin, and its returned abnormally high readings.
one thing i've noticed is certain cars will cause interference with the Fenix.
however, with the polar, my maximum HR tops out in the high 160s on the bike, while i FEEL like my actual HR is MUCH higher. as what is recorded by the Garmin (upper 180s).
i am 50 years old btw.
anyway, for some reason i got the dumb idea to connect the 2 belts with the Garmin pod, and that seemed to give the most accurate reading
i will obviously try the Garmin by itself again and see what happens.