That’s a good question. Aquinas’ De Veritate is
here. Although St Thomas believes that ‘being’ and ‘goodness’ are convertible, he does not believe the same with ‘true.’
Truth is not convertible with being insofar as something could be said to be true which encapsulates some “lack” or non-being (he uses the example of fornication—where we could say it is true that such and such a person is fornicating, but that is an activity lacking in some good) to call a proposition true cannot be the same as saying “it exists.”
Aquinas says, “ An alternative answer would be that in the statement, “The true is that which is,” the word
is is not here understood as referring to the act of existing, but rather as the mark of the intellectual act of judging, signifying, that is, the affirmation of a proposition. The meaning would then be this: “The true is that which is—it is had when the existence of what is, is affirmed.””
To me, this seems like what philosophers call the “correspondence theory of truth”—a proposition is true insofar as it corresponds to reality.
