They have gotten reports of the xp gains in instances already a while back and corrected those once to reflect those in the reference client. Layering and any other exploit is a player issue. Fixing a game such as WoW Classic from how it was developed (old data into newer programmed infrastructure) is a lot different than making a new game that can work out the gate with the infrastructure. They can’t directly pin point exactly where those bugs are at as they have stated before.
It takes time to find them if they are even fixable at all. This isn’t a game they programmed from scratch. They combined both new and old to work within there ecosystem.
Layering is currently still needed for high populated servers. They are absolutely needed because guess what happens to the servers that did get layering removed. Oh look at just a few threads that were made here couple days ago.
Servers that don’t have layering are subjected to have longer queue times or some servers will have a queue when initially it didn’t have one. Again this begs the question on why are you not mentioning the servers that have layering removed when they said they would remove layering?