When I copied the info from MSI Support on your Motherboard, the Ryzen 7 2700x wasn't listed. But as you mentioned IT IS LISTED today. It uses the same BIOS (03/2018) as the Ryzen 7 2700 CPU which is the latest BIOS update.
Here is the new listing from MSI Support for CPU compatibility with your B350 Tomahawk Motherboard:
RYZEN 7 | Pinnacle Ridge | YD270XBGM88AF | 2700X | 100 | 3.7 | 4M | 16M | B2 | 105 | N/A | 7A34v1G.zip |
RYZEN 7 | Pinnacle Ridge | YD2700BBM88AF | 2700 | 100 | 3.2 | 4M | 16M | B2 | 65 | N/A | 7A34v1G.zip |
RYZEN 5 | Pinnacle Ridge | YD260XBCM6IAF | 2600X | 100 | 3.6 | 3M | 16M | B2 | 95 | N/A | 7A34v1G.zip |
RYZEN 5 | Pinnacle Ridge | YD2600BBM6IAF | 2600 | 100 | 3.4 | 3M | 16M | B2 | 65 | N/A | 7A34v1G.zip |
EDIT: Deleted my previous post since they updated the CPU Compatibility List on MSI Support for the B350 Tomahawk Motherboard.