Poor Descent Mk1 Altimeter Accuracy (on runs)

I recently upgraded from a Fenix 3 to a Descent Mk1 (I've taken up diving), and have some concerns around my new device. My understanding is that the Descent Mk1 is based on the Fenix 5 platform, so I was surprised to experience multiple issues which I'd never experienced with my Fenix 3. For example, the granularity of the altimeter, specifically during runs, seems to be far less accurate.

Below are 2 screenshots, taken from Garmin Connect, from the same run (same route, different days) - the top image is from my Fenix 3, and the bottom image from my Descent Mk1. As you can see, not only is the Descent's profile incorrect (missing the climb in the middle of the run), but it's also far less granular (more blocky, less detail).

Fenix 3

I also have multiple examples where the altitude changes randomly during a run, which never happened on my Fenix 3 (in 5 years). See below:

I've also experienced multiple inaccurate (impossible) readings on the Descent Mk1 device. For example, in the below image it read my altitude as 32kms below sea level for a period. I can confirm, I wasn't there.

This device is brand new, and I have confirmed that the port is neither dirty nor blocked. I've also adjusted recording to be per second, and am using GPS and Galileo to try and improve the altitude accuracy. Neither has helped.

It looks like other people have had similar issues. Should I be looked to get the device replaced? Garmin have responded and asked me to do a 20 minute soak, as well as a hard reset. Neither have helped.