This happens to me often and I think this works as intended. As far as I know, you gain war weariness by getting into battles (attacking with you units, not sure whether also when defending) and even much more weariness when you lose a unit.
You lose war weariness every turn (very slightly when at war and faster when in peace). And every peace treaty removes some big amount of war weariness - but this doesn't have to be ALL your acumulated weariness (and you have really a ton of it on your screen), so you have to patiently wait for it to slowly disappear.
Isn't this how it's supposed to work?
(I presume you were in some really long a heavy war to acumulate so many points. If not, then it really must be a bug.)