This is an extant Upanisad quoted from by Madhva (and also Ramanuja in his BSBh. on 1.1.1), but none of the quotes said by Madhva to have come from this source are to be found there.
In this regard, the editors of the Dharmakosa series37 opine (p. 4), based on their reading of the work itself, that the text presently known as Maha Upanisad ``is collected from the original Maha Upanisad, and the [present] text is a quotation from the latter.''