The big picture: Bundling motherboards with processors has become common practice to drive in greater sales. In China, this has been working a little too well for AMD, leading to shortages of several affordable chipsets.
While Intel is having a problem with 14nm chips being in short supply, AMD is encountering its own set of problems in China. However, AMD's shortage of motherboards is a relatively good problem to have. An uptick in demand for entry level boards in has caused A320 chipset motherboards to go out of stock with B450 series boards also becoming hard to find.
Asus and Gigabyte have been heavily affected vendors. In a report from Expreview, it is shown that Asus plans to expand its production in attempt to keep up with increased demand. Even the A68 series from Asus is being tightly controlled to try and prevent an outright lack of stock.
Gigabyte is currently unable to keep up with the demand within its factories. Its current mass production plans do not allow for enough units to be built. Similar to Asus, Gigabyte has extremely few A320 motherboards available at all and still limited numbers of B450 boards. The A68 series produced by Gigabyte are available in greater numbers. Gigabyte is attempting to hold prices steady and is not looking to raise prices unless shortages worsen significantly.
MSI was also mentioned to have raised prices on several motherboards in China due to supply concerns. A major reason for the shortages of lower end motherboards is due to bundling deals. MSI was offering A320 boards paired with the A8-9600 APU but has since canceled the promotion. Other vendors have followed suit in eliminating promotions while supply remains low.
On the high end, X470 motherboards have no supply issues. However, many X470 boards are being sold outside of China. Most have been shipped to the United States and Europe where there is greater demand for flagship components.