I dont think that motherboard overheating. He has 3 Intake Fan Mesh front panel, 1 Exhaust and Mesh Top open...It would take a closed case with 1 exhaust fan to get a mobo vrm to overheat.
You need to give the 2nd image another look. That's an acrylic panel with a mesh filter behind it. There's a tiny strip of grille on the side, but ehh, airflow's still choked.
Then there's one of the downsides that comes with liquid cooling cpus: direct airflow that an air cooler would normally provide is not present, and VRMs naturally run warmer as a result.
The 9900K shares similarities with AMD's old 125w FX cpus: both can throttle low/weak VRM phases.
Also, the system integrators should know by now that Intel's TDP guidelines are a load of crap: that 120mm AIO is only sufficient for the 9900K(95w TDP) at base frequencies. They're technically correct, but in reality... nope.
-Choked front intake
-Liquid cooled 9900KF
-Insufficient VRM phase motherboard
-Only 1 exhaust fan
I don't know about you, but to me, the clues are already there for VRM throttling. I wouldn't be surprised if the gpu was running 80C+ in game either.