Apple Watch Series 10 not recording open water swims

Hi, and thanks in advance for any suggestions.

When I begin an open water swim activity, my watch Series 10 (latest software, paired to an iPhone 12 also with the latest OS), the swim shows as recording and at the end of the swim when I pause it, the screen shows time and distance and other metrics. Then, when I end the activity, it doesn’t record a swim. Instead, it just shows a few thousand steps added plus a few hundred calories burned - so it knows I’ve done something. This has happened twice now. The watch records my runs perfectly and shows a map of the run and all the running metrics. Any ideas as to what’s happening?