I just did a run with an avg HR of 149 and max of 161. At the end I was told a new max HR of 192 was detected, and this synced to Garmin Connect.
Run details can be found here. I'm running firmware 13.21.
Garmin-Kevin this bug is analogous to the same one reported on the 955. I'm in Australia. You have permission to email and view/access my Garmin Connect account.
First, let's not confuse me being in the best shape of my life with me being super fit. I was 45yo when I started running, and I did not take care of myself when I was young. It's just that now, after…
No, it is not a bug. The watch estimates how close to the maximum heart rate you are, and adjusts accordingly.
I thought the same, but Tom Hale posted a link, where Garmin Kevin confirmed that…
I recently upgraded from an FR245, on which I manually set MHR and HR Zones, to an FR255 on which I left on the default automatic MHR detection. Thus far I have only done a few easy base runs and one threshold…
Why do you think this is a bug?
I had a FR 245 before and manually set my Max HR to 186 (calculated it with the Karvonen formula). Then I bought the FR 255, activated it and synced it with Garmin Connect so it had all my data. I changed nearly no settings, so „automatic detection“ for HR Max was active.
After my first run with it, which was 100% in aerobic zone and not even close to my maximum HR, FR255 said, it wanted to correct my Max HR to 183.
Long story short: The watch analyzes data to estimate your Max HR, but that does not mean, you have to go close to that Max HR during that run. In fact, that is a very hard thing to achieve, to really get to your Max HR - which is the reason, why mostly formulas are used to calculate it 
