Why are my personal records not being updated?

Today, I have for the second time beaten my own 5K record. But my Forerunner 955 does not recognise it as a new record. Get ready for some really slow numbers showing how much I needed to start exercising.

According to Garmin, my fastest 5K was done in September last year at 42:08 as part of a 6.46 Km session on a treadmill.

December last year, I ran 5.1K on the same treadmill in 40:23. Garmin measured the run just shy of 5K, and I edited the length in the app to make it match the treadmill. Garmin does not recognise this as a new personal record.

Today, I ran 5.18K on the same treadmill in 39:43. This time, I did not use my HRM Pro+, that I usually use. My Forerunner 955 estimated the run to just over 6K. I calibrated it on the watch to 5.18 as part of the saving process. Garmin still does not recognise this as a personal record, even though it is clearly faster than the 42:08 from September last year. And this time, I even reduced the run length compared to the Forerunners estimate.

Why does Garmin not update my personal records? And even more important. How do I get Garmin to update my personal records? They are neither recognised on the watch, in the app or on connect.garmin.com.