Hello @MrCrowley and welcome to the Community. I moved your post to the Android app forum because it looks like an app display problem or bug. To be complete, was your summary data correct on your Sense 2 screen at the end of your swim?
Laurie | Maryland
Sense 2, Luxe, Aria 2 | iOS | Mac OS
Take a look at the Fitbit help site for further assistance and information.