Hello, I can't be sure what the reason at first directly but please check the below steps. And please check again these two articles I know you already reviewed the memory configuration guide but might be you overlooked something. 

https://dell.to/3Lcshl2

https://dell.to/3LcvO2Q

1. please be ensure BIOS is up to date.

2. reseat the memory modules to ensure they are installed correctly. Check the memory modules or system board for physical damage.
3. swapping memory modules between machines to see if the problem is with the memory or with the machine. 
4. Try to run the system with only one memory module at a time and test each module individually to determine if there are any errors.

maybe memories are not locked properly or manufacturers are different.

reseat the memory again.

if the manufacturer is different but the same manufacturer memory in the same bank

don't forget to check the Frequency.