First of all, keep in mind that the PBE is mainly a place for Testing new abilities, skins, to make sure everything runs smoothly. Actual balance, requires to be tested over several games. More than they can accomplish on PBE, so balancing has to come from live data.
There's isn't much in the way of Thinking, or realistically theory crafting. Basically Riot has to make it's decisions based on the data. If there's error, then it simply comes from incomplete data. This would suggest that the Support Maokai shenanigans never showed up in PBE, or that it's effectiveness was understated. Riot simply doesn't have the man power to test everything in game by themselves. (By definition, beta testing is testing done by someone outside of the original product) But the PBE people can't be feverishly told to abuse test every aspect of the changes because they are effectively volunteers.
Also, too often do I see people theories about certain changes ending up being completely wrong. I would say that even Expecting Riot to be able to predict the outcome of their changes, would be simply ludicrous.