I'm not the only one. I have a friend with similar issue, same Fenix 6 Pro + weekly swimming and the altimeter stop working starting with the update version 24.00 with no beta program but the same period of time when switching from version 23.10 (altimeter was working) to 24.00 (altimeter not working anymore). Now I have Software Version 26.00.

Fenix 3HR Heart Rate Sensor & Step Count not working. Sorry to post under Fenix 5 but there doesn't appear to be a Fenix 3 section. I noticed that my HR hadn't shifted from resting HR for a few days and the green LEDs didn't appear to be lighting up. I have updated the software, rebooted, trawled the internet and sent the question to Garmin
No matter what I do, on any run, my watch always drops a few hundred feet in elevation over the first 1/2 mile to 1 mile. My typical run is 3-5 miles in my neighborhood. It’s completely flat. But every single time the Garmin starts at the right elevation (just over sea level) and slowly decreases to a few hundred feet below sea level.

As we've mentioned, the screen is one of the main upgrades over the Fenix 2, and the 3.1cm (1.2-inch) 218 pixel-diameter display uses a colour system Garmin calls Chroma.

As this has been out for 3 months, if it was causing a new GPS problem the forum would be full of threads on it. Without seeing examples, one common cause of a non-smooth track is being in Ultra track mode so I would check the GPS settings for the activity. You might also want to test different GPS, GPS+Galileo/GLONASS combos. .
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    • garmin fenix 3 gps not working