Someone can only sell you what they own. If they do not own it, they cannot legally sell it to you -- at least not as against a claim of those who do lawfully own it. The people or brokers who sold restricted post split shares into the presplit market had no legal right to do so and the buyers acquired no more than the sellers had a right to sell -- regardless of what they thought when they bought.
The problem lies in the fact that someone sold prior to having a legal right to do so. Which triggered this whole mess.
I suspect the SEC will unwind all the trades and slap a few people on the wrist.
For those who understand no explanation is needed, ...For those who don't none will.