Conventional knowledge would seemingly indicate that finding and fixing bugs happens in-house by the devs themselves, and the PBE is essentially to test balance before changes ship to live. After 4+ years with LoL, its painfully aware that the PBE has nothing to do with determining balance, so you are seemingly correct.
The thing that really blows my mind about that however, is the fact that they make these changes that potentially completely gut a champion or item, or propel it to the tiers of overpowered-ness. Whats mind blowing about that, is that they don't gate these changes from ranked queues at all, meaning that they can completely ruin games for untold numbers of players in ranked before they can start to perform damage control to fix any issues that might exist from a balance standpoint.
The recent changes are especially perplexing, and im sure im not the only one who is starting to seriously question riots decision making capability from a balance/diversity standpoint with how hard theyre shoving AD champions as a whole down our throats at the moment.