Using XMP will always overwrite your manual settings. XMP sets the speed, memory timings and voltages.
What specific memory kit do you have (the gskill part #)??
I would try keeping your CPU at default speeds and then set your ram to XMP and see what happens. Sometimes an overclock or non default setting on the CPU can affect the memory controller. See if your sticks can hit 3600 with your CPU at default Auto settings.
I get home today, though indeed one wouldn't buy an i9 9900K(F) just to run it on stock.