Just to chime in on this for anyone else, I had a serious problem with image tearing when using fractional scaling on Linux (thread here). I also had the external monitor screen flicker problem. I didn’t put the two together.
I moved to two RAM channels (added the second stick) based on some reading around the Xe GPU and how much it likes symmetrical memory. That solved (more or less) my tearing issue, but I didn’t think to check for effects on the flickering. I just tested and sure enough, my flicker went away, too.
It looks like the Xe GPU really likes symmetrical memory, at least with the Linux drivers.