But the Mandate didn't know about it, which I find very strange, considering they are basically the foremost proprietors of history in TTS. No text, anywhere, suggesting that Ishual exists, not even the name, other than on a single map sealed away in a super vault. So not only did the Consult not know about it, but no written historical text of any kind survived that had its name even written.
I will agree that the Dunyain stumbling upon it by accident seems contrived, and I suspect that there are a lot of things about the creation of the Dunyain that we don't know about.