After reading through the other replies and comments, I'm guessing you have a PGA CPU? For example, something like a Ryzen 5 3600X?
I've seen similar symptoms with these chips. Although the initial signs point to your RAM, please try reseating your CPU. Take it out carefully and inspect for bent, broken, or missing pins. (If any are missing, look into the CPU socket and see if it is loose in the respective hole.)
Even if it was working perfectly originally, over time (especially if it was standing vertically with a heavy cooler mounted to it) gravity can take it down and slightly bend some pins out of shape or break the contacts on them. I've seen many CPUs in my shop that simply stop accepting dual channel memory and will only work either in A channels or B channels on a case by case basis. My working theory is that the pins most vulnerable to simple damage over time are RAM allocation data pins and clustered in such a way that these things happen.
Sometimes simply reseating the CPU will address these issues. But sometimes it is not so simple but if you find a bent or broken pins then you have an almost guaranteed diagnosis and don't have to be scratching your head in confusion any more.