So Win10 does a diag first, then decides settings.
If an AMD prob.,then fine(sort it ?), but W10 does have known issues with high DPI(ongoing fix ?).
I heard/knew about the 59-60 prob. long ago, easy fix(as you say, change it).
As with any new OS, learn how it works(do you want a link to 1300+ Tutorials ?)
It's still not a bug though.
Multiple displays with different resolutions, refresh rates, timings and or using different display adapters/connections requires more resources from the GPU, this can move the GPU up into the next memory clock state to compensate and avoid issues such as flickering or corruption.
If all displays are identical, using the same resolution, refresh rates, timings and using identical display adapters/connections then the GPU may be able to run two or more without moving up into the next clock state. It can vary from Bios to Bios and GPU to GPU, but the expected behaviour is increased clock speeds so this is not something we can change, nor is it something we will dedicate valuable engineering resources to.