"Toamna este un anotimp cald și rece!" Is this a natural/common thing to say in Romanian? ("Autumn is a warm and cold season!" sounds strange in English.)
No, it is not. We say "Toamna sunt zile calde și zile reci". We say also "Toamna nu este nici cald nici rece" - to say we have a medium temperatures. The sentence expressed in this way is a contradiction.
The idea is true but the way it (the sentence) is expressed is indeed a bit ambiguous in Romanian too.