I think the advice from Bigglet to try another BIOS is good and something to try. I'm not aware of any directly related RAM issues, but there have been a lot of reported issues with BIOS 1720, mainly around higher CPU voltages and temperatures but also many complaints from over and under-clockers.
If BIOS 1601 is the appropriate step back for your board, then install that and leave all BIOS settings at defaults (which will disable XMP anyway) and try the tests again. If OK, then you might even be able to turn XMP back on
I have had no issues at all with 1601 (or earlier BIOSs for that matter) so worth a try. Because of reported issues, I haven't updated to 1720 and I admit that my RAM is only 5200MHz (with XMP on). But, over the years with various builds I haven't really noticed any practical difference by changing RAM speed. Still annoying not to be at full speed, but sometimes it's better just to be pragmatic and leave XMP off if the BIOS change doesn't work.
Sending back the RAM is an option, but like you say, that's going to be awkward. I'd certainly try another BIOS or disable XMP for a while before you decide. If you do disable XMP, try a couple of benchmarks with it turned ON/OFF. I'd be interested to know what results that gives. I hope one of these works for you. Faulty RAM can't be ruled out, but fingers crossed it's something easy to fix.
Z690 Hero, BIOS 2204, ME Firmware 126.96.36.1994, Corsair 7000X Case, RM1000x PSU, i9 12900K, ASUS TUF OC 3090TI, 2 x 16GB Corsair RAM @ 5200MHz, Windows 11 Pro 22H2, Corsair H150i Elite LCD AIO, 4x Corsair RGB fans, 3x M.2 NVME drives, 2x SATA SSDs, 2x SATA HDs, 2x 4K monitors.
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