Isn't it a bit strange to have both fans and passive radiator on the back?
The heatpipe/radiator are for cooling the CPU. This is 100% passive, dependent on ambient convection. (should get quite toasty in cabinet environments, might force us to adapt some fans on the back).
the internal(infernal?) fans, are there to force airflow in the front-->back direction.
there needs to be some airflow arround the SFP modules, so i believe that the fans should activate in response to (the maximum? / an average?) temperature reported by the inserted modules, and completelly ignore the CPU. (maybe some input from the 25Gbit PHY).
Now, with that much space, i wonder why couldn't mikrotik give us some blowfan-based cooling solution, so it doesn't sound like the router is taking off.