I have the same issue and i couldnt find any way to solve it

I'm also experiencing the same problem. Did you find a way to solve this?

I have no sensors connected, nor do I use the metronome.

Same for me. And no solution. Other than rebooting the watch. But it comes back after a while. 

Exact same issue here. Are you still experiencing this issue, 2 years on?

My cadence switches between 180 and 90 during the run, skewing the average cadence values to 110–130 SPM typically.

I've been in contact with Garmin Support, had several back-and-forth emails, but the only suggestion has been to restart the Garmin. Since then, radio silence. It's quite annoying, as I did used to use the cadence reading to keep my form in check. Historically, it worked fine, then suddenly this issue arose.

Well, I have bought a HRM run as Heart Rate sensor which also gives insight to running dymanics.

With the HRM the problems are resolved. But this does not mean the issue is resolved when only using the watch.

Ah, sorry to hear the issue never resolved. I've been looking into the HRM Pro to get the cadence readings back, among other benefits. Just seems a shame to fork more money over to Garmin to resolve a fault with their product. I might look into seeing whether or not I can get a replacement Forerunner on warranty. Thanks for your response!

same here, since 1st run september '21 no cadence info during run and in Garmin connect, before every run with cadence info

This is still an issue at the moment. A reset of the watch helps for 1 activity and after that the cadence recording doesn't work again.

Garmin suppoer requested access to workout data, but so far haven't heard back.