I have this problem also. I just decided to use a timer for my exercises. Typically I work out different areas (legs, arms etc) 30 seconds at a time during warmup so I thought I would use a 30 second timer and when it finishes, simply press the repeat symbol until I move onto the reps when I don't need a timer.
As you say, the timer disappears before it finishes and I get a black screen. I then have to work my way back to the timer screen. Obviously this is not practical.
Recently, when I was timing my coffee in my French press the 4 min timer was working without going to a black screen. Now, it's not working.
I did a watch power on reset and the problem changed. This time the timer orange ring turned off after 40 seconds but the timer numbers remained visible and continued to count to zero. I can live with that. My watch OS level is: 9.1 on my series 5 watch. My series 8 is on its way.