I had the same issue in 13.15 beta and today also on 13.21
Original report: https://forums.garmin.com/beta-program/forerunner-955-series/public-beta-bug-reports/i/bug-reports---beta-13-15/frozen-screen-during-on-a-swim-workout
Today's activity: https://connect.garmin.com/modern/activity/10047411156 - When I swim I'm watching a time underwater, I swam the screen was frozen at 4:55 I continue swimming and when stopped raised the watch above the water and they unfreeze(without any buttons pressing) and shown 8:24.