AMD Official Mining Drivers
Apparently AMD wants to avoid shortages of their new Vega cards at any cost. Consequently, the company is doing their best to help miners to keep their older GPUs functional for mining. In order to do that, AMD issued an official driver release aimed to improve GPU mining hashrate.
AMD has been working on this driver for a while now. In fact, it became available for download about two weeks ago, though it was an unofficial beta release. Today, AMD posted the same driver on their official webpage.
Radeon Software Crimson ReLive Edition Beta for Blockchain Compute
I have tested this new driver on my rigs and I can confirm that they it does improve hashrate, no more hashrate drop, everything is back to normal. I suspect that the driver reverse the DAG epoch issue, rather than making GPUs mine faster.
I have tried the driver on RX 400 and RX 500 cards, though according to AMD webpage, the Crimson ReLive Edition Beta is compatible with a wide variety of GPUs, starting from the Radeon HD 7×00 series and up to the Vega edition.