Thank you for taking your time explaining the issue, I appreciate it. I actually enjoy the noise profile of these fans, I know I'm a weirdo but I just love the powerful sounds they produce, kinda' like the noise of a powerful engine.
As for the reduced cooling above certain airflow, again I thank you for mentioning it but this was an intentional decision as I'm using the powerful airflow through the radiator to provide a better intake for my GPU.
In order to satiate your curiosity, I actually performed the original ML140 fan test as you requested and they work fine. I'm not gutsy enough to switch them while live though. I already tried swapping the Noctua fans on the splitter as per my original troubleshooting, but that had no effect, the same fan worked and the same fan didn't run below max. Even when running only one of the noctua fans on the splitter, the same fan worked and the other didn't. They both still work flawlessly when on Motherboard.
I do however have a very recent board (x570) with two thermal headers and considered somehow running the fans off of the motherboard, but couldn't think of a decent temperature source. Thank you for suggesting I use the exhaust from the radiator, I might try doing that.