I am a new user of a Galaxy 4 as of March 2022, had similar problems, and can offer two solutions and an explanation.
If you are mainly interested in getting your exercise start and stop times recorded, you can use the "Other workout" option. You have to start this manually and also stop it manually. If you want to record what your "Other workout" was, you will also need to make a note manually.
If you want to use the auto-detection to start an exercise like walking or bicycling, or if you want to manually start such an exercise, you should sign up in the Samsung Health app and enter your age.
I think the underlying problem is that the watch is assessing whether or not you are exercising based on characteristic physical movements of the watch and on repeated measurements of your heart rate relative to a target heart rate that depends on your age. The minimum target heart rate is something like 50 percent of a maximum heart rate that is 220 minus your age. If your age is unknown, a default age of 0 gives a half-of-maximum heart rate of 110 bpm. That is too high, especially if you are older, already in good shape, or both. A low-intensity workout won't get flagged as exercise, and when you are exercising and your heart rate falls below the too-high threshold for 60 seconds, the auto-detect will determine that you have stopped exercising and end the session.
Charles
