Are you using a rockitcool or bitspower IHS replacement or the original?Folks with delid experience,
I recently popped the top on a 12900K for liquid metal treatment. Heated the IHS with hot air station before using Rockit Cool tool for 12 gen. Followed their process via youtube video. No liquid metal is present on the PCB except for the die and IHS. All seemed to go well until I went to boot. The PC failed POST. I found the system runs normally with a single stick of RAM. RAM runs fine at XMP in this system --just not dual channel. RAM works fine in 2nd system and worked with this CPU previously. Different RAM kit has the same result. CPU handles gaming and bench normally for a missing channel. Any thoughts?
MSI Z690-A Pro DDR4
G.SKILL 2x8GB kit
EVGA 850W GA
WD SN850 1TB
Please remove the CPU and check the socket. I know you don't want to do that, but please don't complain about it and just do it.
You either bent pins (good luck unbending them, you need special fine tools and some sort of magnifying glass to fix lightly bent pins, otherwise you are buying a new motherboard), or you have insufficient socket pressure downwards in the socket. Did you re-lid the chip with the original IHS? Relidding the chip should not cause problems like this.
On some previous chips (I know specifically the i9 9900K), you had to sand down the underside edges of the original IHS about 0.2mm if you intended to relid the chip (sanding it down until you saw clear copper along the entire edge was good enough for a 0.2mm measurement if you didn't have tools to get this precise), because the removal of the indium solder would prevent middle section of the IHS from making proper contact with the CPU die, thus causing bad temps/bad core deltas).
Check the back of the CPU. Did you rip off any caps or smd's from either the front or back side of the CPU? If you aren't sure there are many photos available of stock CPU's and delidded front ones so you can compare them.
If your socket pins are pristine with no bent pins, PLEASE highly consider buying a thermalright anti-bend bracket. This will work with delidded chips that have been resealed again and give you proper contact pressure as well. AFAIK, neither the TR bracket nor the der8auer grizzly brackets will work with the rockitcool copper IHS.