Change activity type from Walking to staircase bug

Garmin Fenix 6X sapphire SW ver 19.20 
Since version 18.80 a bug has occurred. I set the long press of the start button with the “Change activity” function.
Bug: As the first activity that I perform I choose "Walking", then after about 20-30 minutes I perform the long press of the start key and select "Stairs". It does not start counting the stairs even after 9 floors, always remaining at zero and not counting them.
If I start the two separate activities, then "Walking" still and safe and then "Stairs" everything works regularly. I also reset the clock without solving the problem.