What is the ACTUAL model of your motherboard, because "MSI B450" is not a model. It is just the brand and the chipset family (B450 chipset), and tells us nothing about what the board actually is. Look on the motherboard itself and the model will be printed on it somewhere.
Also, the BIOS update I posted on your previous thread stands a very good chance of fixing this memory problem as you have the original BIOS version installed and there are like 11 newer versions, half of which are directly related to memory performance and compatibility.
I'd update the BIOS and then reinstall the other memory, and then reset the BIOS to default settings, and then enable XMP, and see where you stand after that. Knowing the model of the memory kits involved would be very helpful in determining why you are having this problem but the bottom line is that most Ryzen platforms, especially those early on or with early BIOS versions, do not like to support 4 DIMM configurations especially at higher speeds.