It certainly isn't always fallacious, but in this context it is, as it effectively begs the question. Observe:
P1: X, Y & Z hold that the Earth is round
P2: X, Y & Z are legitimate experts on the subject.
P3: The consensus of experts agrees with X, Y & Z.
C: Therefore, there's a presumption that the Earth is round.
The point is, here at FES we regularly contest P2 & P3. We are challenging their legitimacy and that consensus, so those two premises cannot (in this context) be used to support the above conclusion. When our position is precisely that the so-called "experts" are wrong, the "legitimacy" of said experts is not a valid premise.