PBE has never been at the center of Riots attention. Plenty of content enters there and still has bugs even when it hits live server. But IMO this has more to do with the fact that Riot doesnt pay attention to how PBE is handled.
PBE is an absolute lawless wasteland. The majority of its playerbase play it not to test new content or bug test, but to actually play league of legends like its a main server where they dont have to pay for skins. The majority of games are surrendered at 15 minutes, so you can barely even check out new content such as the elemental rift changes, dragon souls, elder drake, or even most of the new items before the game ends. Players will also run it down once they have decided that the game is over, since "its only PBE"
Speaking of new items, the new support items on PBE are bugged so that they dont show up in your inventory. So a second bug off of that is for whatever reason since the item doesnt appear on the overview, if you use your pots while laning and hit the 2:45 mark, your team can remake.
Which brings me to another of PBEs biggest failings. Everyone is a huge baby about not getting new content when it comes out. The amount of dodges and trolls that occur when someone doesnt get the newest champ is insane. Earlier today I had 9 games dodged in a row on me because people didnt get Senna, and then the one where I got to play Senna ended when our team remade because of the item bug. Our team remade. Even though everyone was out of base and level 2.
Riot, you really should fix the PBE community, its an absolute joke. In 2019, you cannot link your main account to PBE, you should want player based feedback. PBE should also be far stricter; dodging should lock players out from PBE for 24 hours so as to stop the rampant dodging. Surrendering should be removed from PBE, along with remakes. The wins/losses don't actually matter, its about playing and testing the new content. Intentionally feeding on PBE should be banned far more frequently, especially since players cannot simply make a new PBE account. By linking PBE accounts to main accounts you could also fix the horrible matchmaking of PBE where skill levels are wildly different, and also reward players who make good use of PBE, while punishing those who only abuse it for the free skins.