Just to chime in, I've just went through a similar ordeal although with different hardware config. I've got me a 2x16Gb G.Skill 3200 DIMMs and tried to install them on an Asus Z370E in dual channel mode. Well basically after it refused to boot with anything but me having 1 stick of RAM in DIMMA2 or two in DIMMA 1/2 (single channel) I've started looking for the problem, because the general consensus is that this configuration works fine for many. So, I've cleaned the RAM contacts. I've cleaned RAM slots. I've tried XMP, updated BIOS, tried every slot variation possible, tried over/downclocking, Memtests of both sticks came up fine, then I've read that bent/dirty CPU/MB pins could be the culprit so I took everything out and cleaned everything under an electronic microscope. Then I've moved M2 drive that was ajacent to RAM into another slot. Then I've tightened/loosened screws on the CPU cooler because, according to some, overtightening one screw may tilt the CPU in the socket and some of the pins won't have a contact. Then I've checked the power connector to the mobo to make sure everything is tight and then basically every connector on the mobo. Guess what, nothing helped because of course it didn't. So finally I've pulled the rear panel off and started looking through the wires hidden there, and I've realized that the USB connector that goes from the mobo to the front pannel was touching the mobo in the area where the ram slots are soldered to it and possibly shorting them... So, I've moved it and strapped it with a ziptie. Tried again - PC booted with DIMMs in DIMM A2/B2 and works with no problems since!
So yeah sometimes the bug is hard to squish, I hope you resolve your MB/Ram woes though