If you got a warning with just that one flight, it's probably a bug, and you should put it in the bug report forum.
If you had more flights, received the warning, and then discovered the drastic difference in demand for the opposite direction, then it may not be. My guess is that happened, you removed all flights but one, and you're still seeing that alert? That's not a bug, just the normal way the system works. You get warned, and then once a week the system will check if you've corrected it. So from the time you fix it until the next weekly check, you'll still see that alert on the route, even though you're no longer breaking the rule.