You've just released dengar and he doesn't work properly. Tenacity down doesn't work as it says it does. It supposedly means the target can't resist negative status effects, that is what it says it do...
i know you are moaning about teebo and other stuff but in reference to saying it was an intern who wrote the wrong description, i think it could have been a mistake - tenacity up resists all status effects and tenacity down could do the opposite. The fact that it lands 100% is also strange as no other debuff does that