This happened twice now on my Galaxy Watch 5. When I swim, the swim monitor on the watch would from time to time go back to home watch face display on the watch. I had to apply the same fix twice.
Solution, this is due to Samsung Health Notifications on the watch being turned off.
RednasSosb: This is a very common issue, introduced by the november/december update. You must allow Samsung Health notifications on your watch to keep the workout screen active during a workout.
So on the watch Settings|Notifications|App-notifications|Samsung Health must be turned on.
(You can disable the daily activity alerts in Samsung Health settings on the watch to limit the notifications somewhat if you wish)