Wildly inaccurate walk mileage

[wordlover]

New Inspire 3, generally works fine for past few weeks. But walking mileage seems wildly inaccurate, as if the sensor is completely unable to distinguish between wrist motion while walking from other wrist motions. The most dramatic example was yesterday when my Pixel 5 phone with GPS tracked my walking at 4 miles total, while the Inspire reported fully *8* miles of walking. The general hourly histograms match each other (Pixel/Google Fit app vs. Fitbit app), but the total Fitbit mileage numbers are absurd. Is this a flaw with Fitbit sensors and software, or something potentially wrong with just this particular device?

[wordlover]

Another example today: I walked up and down the steps 3 times, walked to and from the car twice, and spent the rest of the day in the back seat of the car on a road trip, and yet my Fitbit Inspire 3 says I walked 1.5 miles! Is this just how Fitbit's sensor's perform (so I should just return the device as unsalvageable and switch to another activity tracker brand), or is this a problem with the particular device and I should get it replaced?

[wordlover]

And no reply at all from Fitbit. That tells me a lot.