To quote a Reddit user named onelamefrog
As someone who has worked QA for a major game company, I'd like to say a few things.
This happens all the time. Almost every bug you encounter in a retail game was known. Most of the obvious ones anyway. Of course there are bugs QA didn't pick up, but a million players tends to make bugs come out you'd never expect or could possibly design a test case for.
The problem is expectations. There are deadlines that have to be met and basically fuck all the small bugs that don't majorly destroy the game. Just ship it and fix it afterward if it's especially egregious. Basically - most of these bugs are not worth the effort or can easily be removed later. The important part is getting the game/content patch out on schedule to avoid negative public perception (believe me, a few bugs are much better than a delay) and extraneous fees (such as certification).
From what I've seen it looks like Riot uses the PBE for stability testing. That means they push the content out solely to make sure it works with the system. Their main concern is that new content doesn't cause the entire game to crash by having people use the content in a running system. If there are any gamebreaking (as in the game stops working at all) bugs it is a huge concern and will be fixed before live.
That being said it's still not that simple. Yes, Riot will ignore some bugs in favor of ironing them out later. Still, it's impossible to account for every bug and fix them immediately. Fixing a bug requires 1) reliable steps to reproduce them 2) a programmer/designer who even knows where to look 3) more testing. Without enough information they cannot even attempt to solve a bug, if they have the resources at the time at all. Your average user will write something like "game crashed when bought Tiamat" when the actual bug may be putting any AD item in slot 3 at 25:00 minutes on a specific champion. If a profressional QA tester doesn't catch it, it's incredibly unlikely that 12 year old Timmy will track it down, write accurate reproduction steps in a literate manner, and fully report the problem with detailed descriptions to Riot.
With not enough information about the bug, it may slip through, be pushed through anyway, or even become a bigger problem with live data - depending on what caused it. Pushing from closed to live system isn't a simple task either! They're not the same thing! You can't sustain a 1:1 clone of both systems - which would be ideal, but impossible with Riot's scope. So there's differences you compensate for when patching to the live system. This may introduce or exacerbate new bugs. For example the current problem for Fiora - missing tooltips. Someone just plumb forgot to add those changes into the patch. It happens.
TL;DR: PBE is for stability testing, bugs can even pop up after the fact, and the amount of information from PBE is insufficient to catch all bugs. Companies are likely to take fewer losses (monetarily and public perception) by just pushing the content out.