it was about heart break or it is about heart break?

Mixed tense are often used for facts / ideas.

1) Originally, it was thought that the Sun revolves around the Earth.
2) It was discovered many years ago that the Earth revolves around the Sun.
3) This research, from 1900, shows that ... (some fact)
4) The movie was a hit because it is about heart break.

1) Using revolved (better in my opinion) reinforces that the fact was incorrect and people know better now, whereas revolves emphasises what people at the time thought.

2) I would not use revolved (unless the Sun explodes or the Earth gets moved and the fact is no longer true).

3) Using showed is also fine, but (for me) using shows reinforces that the fact and its implication is still important and true whereas using showed highlights the difference in fact / implication between what was thought before and after this research.

4) Using "it was" instead is also okay (to me), but "it is" describes the movie (it is still about heart break) and so I would use that. Using "it was" suggests the people at that time had a positive reaction to it, which is good, but perhaps that people today might not (perhaps acceptable norms change or it is not shocking enough anymore).

With the use of past tense, the speaker emphasizes more on the EVENT of watching the movie which happened in the past than the thought the sentence expresses(interesting experience/movie about heart break) !

"That movie was an interesting experience because it was about heart break."

On the other hand, If you want to put less emphasis on the actual event of watching, Rather, you want to put more emphasis on the concept/theme/idea which the sentence is trying to express(interesting experience/movie about heart break), then you should use present tense in the both clause!

Use of present tense on both clauses makes the original thought of the sentence more dominant than the event of occurring the event in the past!

"That movie is an interesting experience because it is about heart break."

It depends on you, what you want to emphasize more, you can use past or present accordingly in both clauses.

N.B. Using different tense on different clause makes it ambiguous to understand the speaker's thought!

The first half of this sentence uses past tense ("was") so it is more natural to describe the contents of the movie in past tense also, but "is" can sound quite normal if the rest of the sentence also uses present tense, such as: "This movie is interesting because it is about heart break."