There is a text like this,
"The belief that the world was round was not strange to Columbus."
I think 'the world was' should be fixed like 'the world is'. Because 'the world is round' is the truth of all time. Am I wrong?
With a timeless property like that, your version (with is) is possible; but it is not usual.
The habit in English of converting the tense of reported speech in the past ("He believed 'It is round'" => "He believed it was round") is so strong, that it tends to overcome logical objections like yours.