Peter Tan a.k.a. Shamino, legendary overclocker and employee at Asus, and Matthew Hurwitz, Product Marketing Manager at Gigabyte, said that AMD had given the order to pull the AGESA 126.96.36.199ABA microcode due to a technical bug.
The AGESA 188.8.131.52ABA microcode was supposed to solve the specific issue of Destiny 2 not loading for Ryzen 3000-series owners and booting problems with certain Linux distributions. However, it looks like AMD might have jumped the gun on this one, and a bug slipped through.
Tan has commented on the Asus' ROG forums that the main reason for scrapping the AGESA 184.108.40.206ABA microcode was that it somehow downgraded the PCIe speed for the BXB-C from Gen 4 to Gen 2, which in return caused system instability. Now that AMD is aware of the problem, a fix will probably arrive sooner rather than later. In the meantime, motherboard manufacturers are on standby until AMD provides the updated microcode for validation and implementation in their firmware.
The Ryzen 3000-series processors hit the market a little over two weeks ago. Motherboard manufacturers are on a tight schedule to release new firmware for their existing AMD 300-and 400-series products. Bug detection requires thorough testing, precious time which motherboard vendors, unfortunately, don't have at the moment. As Tan has noted, sometimes it's a lose-lose situation. There's a public backlash if a company rolls out a firmware too quickly and overlooks a bug or if it takes too long to release one.