ML240L = low performance model. The ML240R is the high performance model and much better.
Understand, the i9-9900k can hit 250w with stock values, which is about the limits of that cooler. The ML240R is closer to 300w limits, and most 280mm are 300-350w.
As you approach the limits on any cooler, air or liquid, it becomes less efficient, and temps go up faster on the curve, until you reach the coolers limits and the curve goes almost straight up.
The 9900k staggers its thread usage and power consumption according to temp. As temps get higher, the cpu will not use a portion of its threads. Intel did this to protect the cpu, since it is such a monster. You won't be able to use the maximum amount of cores/threads unless you have the cooling ability to so so. You do, but barely, so you can expect higher than average temps. What you have is basically the equivalent of a cooler that's 1 step up from what would be considered a stock cooler.