It may have something to do with the version number and the chip set brand, check the list...
The motherboard's QVL list will tell you which RAM is rated to run at which speeds. It can be found on the support page for your motherboard.
From ASUS B350 Plus Support : PRIME B350-PLUS Driver & Tools | Motherboards | ASUS USA .
Latest BIOS & AMD Chipset Updates:
BIOS - 3806 - 03/27/2018
AMD CHIPSET - 02/05/2018
Your BIOS on your Motherboard for the Ryzen 5 - 2400g must have this version or above installed:
Ryzen 5 2400G (3.6GHz, 45 ~ 65W, L3: 4M, 4C) ALL 3203
For Memory from QVL , this is the RAM that you are using:
CORSAIR CMK16GX4M2B3000C15（Ver5.39） 2*8GB SS N/A 15-17-17-35 1.35 3000 3000 ●
Should show 3000 speed since it is supported by the Ryzen CPU processors.