This isn't making much sense to me either. If the system builds pressure, which apparently it does, it will be by way of the relief valve and heat will be generated accordingly. So it's no surprise there is heat. But I would expect it to be pretty much the same whether the fins are being used actively or not. I guess if the fins are moving, some portion of the hot fluid is diverted out to the actuators with cooler oil coming back, so I can see it having some cooling effect, but I would not think a lot.
The only explanation I can come up with is that your system is right on the cusp of overheating, and the cooling effect of operating the actuators is just enough to keep temps within range.
I see there is a filter in this diagram. Has that been changed recently? Perhaps it is generating back pressure and producing additional heat?
Otherwise I think I'd focus on the seawater cooling side of things, especially considering it's propensity for fouling. What is your source of cooling water? Is it s dedicated cooling circuit with thruhull, strainer, pump, discharge? Or is main engine cooling water diverted to the hydraulic cooler? If main engine water, is it drawn off before or after it cools engine components, all of which will in turn heat the cooling water. I downloaded an RS400 manual and it said the cooling system is designed for max 80F water temp. You can tell Wesmar is based out of the PNW
What is your sea water temp? Have you ever opened up the cooler, removed any zinc fragments, and rodded out the cooling tubes?
Oh, and I forgot to ask what is probably the most important question. Was this previously working fine, and recently started doing this? Or has it always been this way? Or this is the first time you have used the boat this way so might just be discovering it for the first time?