I have done some investigating and figured some things out.
When you first launch the game, if you turn the grind setting down to 0, it will work properly. There are a few things I've found so far that trigger the bug.
- Complete any Daily, Weekly, or Seasonal Challenge. The moment that the challenge notification pops up, you will be back at 100 grind assist.
- Play any mission. As soon as the mission loads you will be back at 100 grind assist.
- Make any purchase or item unlock from the shop/skate pass. (Maybe this one will make them care more since it's shop related)
There are probably more triggers for this bug, those are just the ones that I noticed first and are most annoying.
Going into the options and "wiggling the switch" fixes it. Go to 5 and back to 0 for example. But it's going to break again as soon as you do any of the above things. That's why it feels like the setting turns on at random, you probably completed your trick of the day challenge or something.
Lastly, fast traveling doesn't seem to cause the bug, so using the map is still safe.
Edit: Replay Editor seems safe too. If anyone finds more triggers for this bug I guess add them to the thread.