Apple Watch counting too many flights climbed

My apple watch (series 4) counts too many flights even if I'm just sitting at home.

The watch has been already sent back and replaced for free under warranty period, after few months the issue came back again.

I followed all the instructions given by Apple support (let the battery discharge completely, resetting, recalibrating, repairing....) but problem is still there.

Apple store proposed me to exchange my watch with a series 6 paying the difference, but this could make sense if the issue affects series 4 only (and nobody knows).

Anybody had the same problem on other series?

Is there a real fix available?

Thanks