Wow, just wow...
This bug was privately known by Riot for about 4 months and never publicly announced or was known about until now?
The fact that you asked if Riot is "hiding" champion bugs from us is just sad...
You do know about that Kindred bug in LCS last year that killed that poor Lee Sin with 50% HP on a third attack on her Mounting Dread (E)? That bug was there since Kindred was released but it was super rare and almost never happened, like 99% of the time. The only reason Riot finally knew it happened because it was captured on video, in LCS, which launched an investigation. How many bugs did Azir have on release and Riot was deploying a fix with every patch for him?
It's not about Riot "hiding" the bugs from us, it's about first realizing there is one, replicating it internally, finding out why it happens and then deploying a fix. Is there a reason why would Riot "hide" the bugs from us, other than your "Riot is a shady business" argument?
Sometimes a bug may be giving an advantage in game that's not intended and depending on the severity (and how easy it is to replicate), a champion might be disabled until it is fixed. Or if it's quite hard to replicate, no one would be the wiser until Riot fixes it, at most during the next patch. Oh, and the best part is that at the end of each patch notes, last notes are always about various bug fixes that most people browse through because they're not interested.
The point of a PBE is to primarily test new content for bugs and Riot fixes a lot of problems for the actual Live release. But again, a bug is nothing more than an error in coding that causes unintended things to happen. No one programms a bug into a game, they just happen and bring along unintentional functionality that may be bad for the game. Sometimes, bugs may even expose better functionality that may be fixed by bringing that functionality to Live but i don't think that has happened yet.