Both are correct, but they mean very different things (if sentence 2 is meant to say 'had been submitted...') The first means that I found this news out, and I found it out on the 28th. In other words the tender may have been submitted five days ago or five years ago!- but *I* found out on the 28th. The second means that the *event ITSELF* took place on the 28th: in other words whoever submitted the tender did so on the 28th.

They are both wrong. "had been" is probably correct, on the basis that the submission was made before the speaker was informed.