Now, first off, if you mean the mechanic is a bug, that's not.
He hasn't moved, attacked, or used an ability, so why would he become visible?
If you mean it's a bug solely because it could be used to dodge LeaverBuster:
That would mean you would have to play Teemo, get Sunfire or some AoE burn effect item, and a suitable jungle to target.
Then, you just stay there.
Now, you have to hope that your enemy team apparently will never ever go near that jungle.
If they do, it's fairly obvious you're there, and they'd either use a ward to reveal you, some item or ability to reveal you, or some sort of general attack that would hit stealthed enemies.
Enough times, and you'd still die, then end up back at base, where LeaverBuster would check.
Going through this just to AFK and dodge LeaverBuster seems not worth it.
Also, if it's really that much of an issue, Riot could just check to see whether Teemo players with burn items like that have not moved for X minutes.