It’s IMO more interesting to look for reasons that something could be consistent than try and point out issues like this. Still suits could have some ‘sci-fi’

It’s IMO more interesting to look for reasons that something could be consistent than try and point out issues like this. Still suits could have some ‘sci-fi’ scale thermal battery assuming Arrakis gets cold at night. You could get warm enough during exercise the body starts to sweat while the suit generally feels cool in the sun and sufficently warm at night.