Ah now that explains a lotI have a Nitro too, and just had a look at how the cooling is arranged; the heatpipes split up into two groups of fins at either end. In the middle there are almost no fins, and it's partially blocked by the logo on the shroud too. Thus it's no surprise that the middle fan doesn't improve cooling.
This seems to be quite common with triple fan coolers. The Fury that I had before the Vega 64 had a middle fan blocked almost entirely by the power connectors and the shroud. The Radeon VII that I am using now has the middle fan partially blocked by the logo on the shroud.