In order for Riot to fix bugs (which they do almost on a constant patch basis), they need to be noticed and reproduced. One instance of a bug does not mean Riot immediately knows how. when and why it has happened and can implement a fix for it. Certain bugs require of champions to be disabled (if it is an exploit and can win or lose you the games and also depends on how easy it is to replicate) and certain bugs require to be hotfixed. Hotfixes are reserved for immediate problems and have their limits, because changing code on the fly may end up introducing more bugs. These have happened in the past. Riot knows this and is thus reserving to solve a bug in this manner only when it absolutely cannot wait.
For example - one LCS game, Kindred managed to one shot a half HP Lee Sin and that was caused by minion damage that needed to happen in a very specific, very short time frame of her casting W and having 2 stacks on him already. That minion damage greatly amplified Kindred's third attack and translated it into ability damage. That bug probably existed since Kindred's release but since it was very rare (and we were very lucky we even got LCS footage of it), no one knew this was possible until everyone actually saw it happen.