It’s not just a translation issue, but that the composition is just archaic. The constituents had real meaning a long time ago when they became a word, but today you just understand 天真 as its own word and move on. If you didn’t know that word but understood 天 and 真 you wouldn’t be able to figure out the word’s meaning.

The same is true with many English words as well actually: many of them started out as composites that were meaningful in the last but today are not. For example, “understand.”

Yes, 'heaven' is more precise. To reflect 'heaven' usually requires a deeper grasp of the language to twist it a little bit closer for its real meaning. And your point is great - the concept is hard to be translated into another language. For example, there are so many Buddhism concepts which is very hard to translate.