I have the new Galaxy Watch 3 and regularly use the timer in countdown mode but 50% of the time the timer just stops and becomes inactive gor no reason. An example of this is I set the timer for cooking to 17 minutes and I go and sit down and look at my watch to monitor how long I have left. A temp watch face indicates there is an active app. All of a sudden the temp watch face disappears and the timer is not showing any more. Last time I had to guess a new timer but today the timer went off correctly.
Are you using the Samsung timer app on your watch?