Found this thread on Google.. looking back at the timestamps it is safe to assume that nobody at ASUS gives any sh.. I guess. Here's a short summary if more people stumble upon this post:
ROG Ryujin is a "smart" cooler -> someone at ASUS thought it would be a good idea to connect the fans directly to the pump instead of the mobo, so it seems like a "full-software" control. This software is Armoury Crate (Devices -> Ryujin -> Fan Control). When the CPU is relatively cool, the software stops the fans (both the radiator and the "micro fan" sitting on the pump) completely, which is OK for light loads. The problem: it often "forgets" to turn them back on! Maybe some monitoring service just exits at some point, just a wild guess.
Use-case #1: I got back to Desktop after a gaming session and saw that the CPU (Ryzen 9 5950X) was approaching 90Â°C! Fans were standing still for hours.
Use-case #2: I installed some software, needed multiple reboots, at some point the system wouldn't boot - CPU thermal alert! You guessed it, the fans on the Ryujin's rad were standing still.
Use-case #3: uninstalled Armoury, had trouble re-installing, in the meantime the CPU was hitting 90Â°C again due to the fans standing still...
Windows 10 Build 19042.746. The question is: does ASUS really not get the fact that they produce one of the crappiest software in the world in terms of stability and bugs? How can a sane person give control over CPU cooling to such a software? I guess I'm done with that one and will just connect the rad fans to a CPU_FAN header.