I updated from bios 1304 to bios 2004 on the Z690-G GAMING WIFI with a 12900k at stock and my XMP got broken too. I was experiencing crashes in certain games in 15 minutes or so consistently, some applicaiton hangs and some bluescreens with "MEMORY_MANAGEMENT". Disabling XMP allowed me to play the games which crashed before for over two hours with no problems. I ran memtest86 with both XMP on and off and in both cases it passed with zero errors. My memory is Corsair Vengeance DDR5 64GB (2x32GB) 5200MHz C40 Intel Optimized Desktop Memory so its a very modest XMP yet it still was broken. It was rock solid stable for six months since I built my rig.
I should also say that in memtest with XMP on the RAM was getting up to 53-55C max (usually at 50-52) and with XMP off the memory gets up to 49-51 in games and a little less than that in memtest. Are those temperatures OK? Like I can't imagine it was getting above 60 in games with XMP on.
I have pretty much the same hardware as you, 12900K / Z690-G Gaming WiFi M-ATX / Corsair Dominator Platinum RGB DDR5-5200 C40 64GB.
Using XMP I profile with no issues on Bios 2004.
Do you have Anno 2205? That game was by far the easiest way for me to crash my system with the XMP enabled after the bios update, I only had one other crash in Warhammer III after like 3 hours of gameplay. As I mentioned earlier I did run Memtest86 after first seeing these issues and it passed a full 8 hour test with zero errors with XMP enabled which really confused me while troubleshooting. I wonder if there is any substantive difference between the Dominator and Vengeance kits at 5200-CL40.
I also had to disable XMP for system stability. I had put in 1704 on my system. It was unstable. I then put on bios 2004 thinking it would correct it. It wouldn't then I noticed I could not go back to the one prior to 1704.
I was getting random BSoDs. Memtest86 would randomly fail in different places and then pass once in a while. This when away with XMP being disabled.
System is 12700k, z690-g, CMK32GX5M2B5200C40 corsair vengeance 5200 32 gb kit