Heart rate high intensity - Fenix 5s

No, it applies to Optical Heart Rate Devices generally. The issue is not so much high intensity exercise, but rather intervals. OHR sensors tend to have problems tracking rapid changes in heart rate that are associated with interval training, and once they lose your heart rate, can have trouble re-establishing it, and can lock onto rythmic movement rather than HR. 105 sounds suspiciously like the tempo you might move the handles on an assault bike during an all-out effort.

Using a heart rate chest strap is probably the best (but not cheapest) solution. The f5 series work with both ANT+ and BT chest straps.

A more gradual ramp-up in intensity may help the OHR remain on track with HR, although it would be fair comment to say you shouldn't have to change your workouts to suit your device.

You could also try positioning the watch on the fleshy part of the wrist away from the wrist bone - this would make it less prone to movement as your wrists flex with the handle movements of the assault bike. Ensuring the band is snug can also help - I find I have to have the band one notch tighter for accurate measurement while running than I do for walking.

Thanks mcalista - Great notes - I have started slower to see if it would pick it up and build up to the heart rate but nothing. Another note: If I go for a run with the correct reading and then switch it to Cardio and hit the assault bike it picks up a better measurement. It's super frustrating as I don't always want to go on a run to kick the HR into gear. I also have tried flipping it around on my wrist, putting it higher on my arm, and even on my ankle. Nothing seemed to work. For a $900+ watch, you'd think they could correctly measure the HR in all workouts...

Do you know if the only difference between the Garmin HRM - RUN and HRM-TRI? Is that the TRI is waterproof? Trying to figure out what I should buy or if I should switch to a different watch. Also, will this band work with doing interval training in which I do not wear the watch? I.E. something in which I do Olympic lifting and assault bike.

hmm, that's odd that starting with a run will work, but a gradual ramp-up on the bike won't. Just an obvious point, but are you getting a solid HR lock (ie. the HR symbol goes solid) before pressing Start on your workout?

I'm not convinced switching to another watch will solve the underyling issue (many of them use the same underlying Valencell HR sensor). But if you can buy from a store with a good no-questions-asked returns policy, you don't have anything to lose by at least trying...

ANT+ radio signals don't travel underwater, so the HRM-TRI stores the HR data, then forwards it to the watch when you are out of the water. This is the key difference between the HRM-TRI and the HRM-RUN - the RUN is real time HR only, not store-and-forward. For the use case you are describing of not wearing the watch, you want the HRM-TRI.