It's quite an oddball indeed.
Basically 65% is the "new 100%".
Problem is : 65% reads, looks and feels harmless - by all means.
There isn't even a yellow marker or something, neither does the colour i.e. change to Yellow or any Caution Signals popping up when you approach/pass it. Nothing that would have catched my eye.
So I think it's safe to assume that folks i.e. first see the significantly reduced heat figures operating in Normal Space and SuperCruise, thinking their Ship's Heat output has simply been "nerfed" and think they now got more headroom to play with.
That will need some refining in order to explain why Ships now start irreversibly overheating (unless you take positive action) at
65% heat, with both HUD display updating rather slow, temps creeping up continuously and smoke beginning to come up from the dashboards