"log HRV" are also enabled on my 5x but no change, my HRV Stress is always 1.
Anyway not a great problem because I use the Android app Elite HRV that work very well.
All of these tools, like HRV, Garmin's suggested recovery time, even your own training log can only be used as individual data points that inform a consciously-executed training program. There is no single "silver bullet" test or tool that can remove the subjective element of physical training. The trainee must pay attention to their body and be an active decision-maker in any training program to have optimal results.
Heart Rate Variability can be an accurate marker for the physiological stress of a training load, but it has limitations.
It doesn't always reflect injury or many types of illness. It can be affected by medication. Even if you use the test at the same time every day in the same way, it's naturally going to vary from day to day, even based on your emotional stress levels.
It can still be helpful as a planning and decision tool. If you have a hard workout coming but you aren't sure if you're quite recovered for it, this could push you one way or another. If you're coming off an illness and think you're ready to run, but your stress level is still reading Very High, you might wait a day before resuming your training.
If you understand what the test is actually measuring and how it relates to your body's recovery process, it can certainly be used for more than just curiosity. But you're certainly right, on its own it does not predict performance with perfect accuracy.