Big heart rate problem while biking

A nylon strap instead of silicone did wonders for me... Or buy a chest strap.

The problem could be sweat, dust or  bumpy terrain 

Sounds like a connection problem rather than optical/ fit problem.

Looks similar to this software issue forums.garmin.com/.../has-9-33-9-36-ruined-heart-rate-monitoring

I have the exact same problem. HR drops from 140 to 80-100 and stays there for 20-30 minutes and then ‘resets’ to normal figures. Happens for me only when I share my hr to edge 1040 solar. Starting a swimming activity doesn’t have these strange droppings 

Normaly i broadcast heart rate without starting an activity. Today i did exacly what you suggested. Same problem my heart rate drops from 140-145 to 70-80 for about 4 minutes and then doubles to 140-145 again. Really pushing the past doesn't effect the heart rate. Never had this problem before 9.36

I have the same problem with software 9.36 when monitoring bike activity, at first it detects exact bpm and then it drops from 140-150 bpm to 90 bpm. I tried to reposition the watch but it doesn't fix.

The solution is not to buy a chest sensor, because if I spend money on a high-end watch like a fenix 7 I expect it to detect bpm well.
I bought it in May for a few months now that I monitor different types of activities 4-5 times a week. So it's not a problem of position on the wrist or vibration with the bike.
I don't care about the accuracy of a chest sensor, I care that the watch does what it can do well.

Before the latest 9.33 / 9.36 software I didn't have this problem, the watch is positioned and tight as ever.

Edit: 

I forgot one thing: this is not a problem with any optical wrist reader. Before the Fenix 7 I used an Apple watch and used it to monitor the same activities that I monitor with the Garmin. Strength training, cycling, hiking, swimming, so the same stresses. It is not specific in sports like Garmin which is clearly superior in many things, but speaking of bpm it has always detected them without having drops during obvious and continuous efforts. So for me it makes no sense to say "you know that the optical reader on the wrist is not good". It won't be as accurate as a chest sensor, but giving me 90 bpm instead of 140-150 is too much

I hope they can do something

Glad I'm not the only one with this issue. The solution is not buying a chest sensor but Garmin fixing a problem that didn't exist before.

To reiterate, this problem was introduced with the current or maybe previous fw updates. Up until say 2 months ago the heart rate / watch 1030 connection was just fine.

same problem here: https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/fenix-7-series/327218/the-problem-of-measuring-heart-rate-while-cycling/1588740#1588740

it would like an official garmin statement... with the garmin fenix 6x everything worked fine even without the chest strap...

And you're not using a chest strap that's your problem, period end of story. You CAN NOT get an accurate optical heart rate reading on a bicycle while your wrist is bent and your watch is having movement/contact issues. It's damn near impossible. I don't care what anyone says, chest strap...