I am a recreative swimmer and I am testing Apple Watch. When I am swim with Garmin there is a manual lap option and for instance fastest I ever swam in 50m pool was 1:17. However with Apple Watch, and its auto lap option (there is no manual lap), it reports 1:14 and without effort. I can go down to minute. I mean I wish that I am in that shape, but it seems something is odd Lap count is correct and swim styles are properly detected on Apple Watch. I checked other swim apps, but they do not offer manual lap.
An overly optimistic pace is often due to delay in detecting movement, so it starts the timer mid-length. Pausing/resuming doesn’t fix this as detection may still lag. A strong push-off helps detection. But don’t stress about a 3-second difference per 50m—just track trends on the same watch to gauge progress.
Probably that is the issue, but Apple Watch is an expensive watch to argue “ok couple seconds off” should be fine. Especially with Ultra. For more serious swimmers they have even longer dive in the water.