Based on your description of the circumstances, and that you (the DM) support it, the answer is an unambiguous Yes according to the D&D 5e rules.
Granting advantage happens two ways: a mechanic says so; and at the DM's discretion, to handle the thousands of things that come up during play that the rules can't predict (PBR p. 57 / DMG p. 239).
From the DMG section on Advantage and Disadvantage (ibid.):
Advantage is also a great way to reward a player who shows exceptional creativity in play. [...] In other cases, you decide whether a circumstance influences a roll in one direction or another, and you grant advantage or impose disadvantage as a result.
The bulleted list that follows reiterates that granting advantage is within the DM's discretionary power and responsibilities. The example on the following page is relevant and worth studying too, as it describes the DM granting advantage and imposing disadvantage entirely due to circumstances, rather than because a mechanic grants or imposes them.
The emphasis at the beginning of the section that this is one of the most important tools in your DM toolbox underlines that not only is granting advantage at your discretion, but judicious use of this discretion is a vital part of DMing 5e correctly!
Do you want it to grant advantage, as the DM? Then it does. Voilà!
Your sense that this shouldn't keep on being quite so advantageous after the first attack is probably wise, though. The simplest ruling I could see making there is to re-grant the saving throw on each attack (successful or otherwise — the victim can see how the attacker is moving differently than they ought). The advantage shouldn't last too long then, unless the opponent is quite unlucky, or their chance to save is low to begin with and their confusion should last.
You might also consider on a case-by-case basis whether, at any point, the circumstances of being attacked in this manner might grant advantage to the defender for saving to disbelieve this unbelievable circumstance. That's under your discretion too, of course!