Hi Ducky1,
Nothing is completely waterproof and even the air that gets into the mattress will have some moisture in it that will condense and can accumulate and with a less breathable cover would have a more difficult time getting out.
It may be more than you want to read but you read about some of the many factors that affect temperature control in post #2 here and in post #29 here.
The Pristine would be more breathable than a “membrane” type encasement so this may be part of the reason why and the cotton also wicks moisture very well (although it doesn’t store it nearly as well as wool). Each person can be very different as well though and what is warm for one may be perfectly fine for another. Natural fibers are generally better at regulating temperature than synthetics. Your own experience is certainly more important than any “theory” though because it “cuts to the chase” and says more about how all the layers and components of your sleeping system work together in combination than analyzing the properties of each layer separately which can be more useful if you need to make changes to cool your mattress down.
Since latex is the most breathable of the foams (probably more breathable and temperature regulating than the comfort layers in your big cloud like puffy mattress although I don’t know what they are), and you have already done well with the Pristeen and the cotton quilted cover over it, my guess is that you would very likely be fine in terms of temperature regulation with the same combination as long as you are OK with how they affect the “feel” of your new latex mattress.
Phoenix