I did a pool swim yesterday and noticed that the calorie count was a bit higher than I would have expected, but didn't think anything of it. Then checking the details on Strava and Garmin Connect, there is a random pause of 36:24:28 in the middle of the activity that completely throws off the synchronisation between laps / heart rate / distance. Looking into the TCX file exported by Garmin Connect, there's a lap marker inserted in the middle with a duration of 131068.75 seconds that starts 3 seconds after the beginning of a lap that miscounted the distance (150m instead of 200m for four lengths), but even though I've seen the watch missing lengths before I've never seen the time tracking bug out like this.
I had to do a hard reset on the watch last week due to the GPS boot loop bug, and this seems like the kind of issue that might be caused by either a bug in the firmware or some kind of corruption in the file writing that I hope wasn't triggered by the reset..