Hi Dave!
Yes, I was a little surprised about that. I have a different cooler cap which opens at 1:00 bar instead of 1:50 for safety reasons. I once had a broken hose, which I blame to age and high pressure at very high load (temp was higher but ok). I also run the coolant level on minimum. There is a lot of coolant in the engine (11 litres) and if you calculate the volume gain from 20 degree C to 100 C, the volume of the reservoir must be to small to cover that without a build up of high pressure. There must be a volume gain of almost one litre.
I could not explain why, but maybe this is a part of the „good“ temp.
Nic