It should not break attunement
The key to this answer is the following from the Polymorph spell description
The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast. It retains its alignment and personality.
What does that italicized part mean? For a given creature, it can be rendered as "you retain your alignment and personality."
In plain English, since we don't have a further game-rule-specification of what 'personality' means, you are still you, thus you are the creature that attuned to the magic item. For a temprorary period while you are polymorphed, what you are (beast rather than humanoid) has changed but who you are (you, with your personality and alignment) has not.
As @ThomasMarkov pointed out in paragraph 3 of his answer, this is worth discussing at the table so that you are all on the same page since there is some ambiguity present.
Good reasons not to have polymorph break attunemnent ...
Point one is what ThomasMarkov mentioned on how this would make the game very messy and fiddly each time the spell was used.
Point two: I'll add to a theme from a Crawford's tweet responses over the years: if polymorph broke attunement it would say so in the spell text.
Attunement, and specifically the attunement to no more than three items, is a core game balance issue for this edition.
A third situational reason is from @Yakk's answer: breaking attunement on a cursed item (like the Sword of Vengeance or Berserker Axe) would just "happen" if you got polymorphed. (RAW, they require a remove curse to do that).
Point four is this:
The target’s gear melds into the new form. The creature can’t activate, use, wield, or otherwise benefit from any of its equipment.
What the magic of polymorph does do is also transform the magical item into a part of the beast; that's what the spell says it does. It doesn't say "and it breaks attunement." That closer affiliation of your gear, magical and otherwise, to the wearer while in beast form argues against it not being attuned by the temporary effect of the polymorph spell. (True polymorph is once again a similar but different case since its effects can be made permanent/until dispelled).
In one of the first UAs WoTC put out, they cautioned against messing with (1) attunement, and (2) concentration on more than one spell (or similar effect) at a time. Those were both significant balance issues in this edition based on lessons learned from previous editions. (There's a third thing that at the moment I can't remember).
There are some related Q&As that deal with the similar issue as regards true polymorph to include this one, where the explicit point on the soul (which comes via different spell, astral projection) addresses "Who is really in there? Is that a prince that was turned into a frog?"