This is a good question, and I think it’s best to go to Saint Thomas, as I’m not actually intimately familiar with how he explained the change.
newadvent.org/summa/4.htm
Sections 73 - 83 concern the Eucharist. 73 - 77 are probably all profitable reading regarding the issue here, and earlier portions serve as a foundation for later, but I think Thomas addresses such objects under section 77 (accidents).
Essentially, it seems that the accidents are preserved by the divine grace of God. While not possible in the ordinary nature of things, it can be made possible by God.
This is not the same as a square circle. Or perhaps to use Aquinas’ own example, a (euclidean, which for the remainder of this example, will not be repeated) triangle with angles adding up to less than 180 degrees. It is essential to a triangle that it’s angles add up to 180 degrees. Therefore, for a triangle to exist with angles less than 180 degrees, it both is and is not a triangle at the same time. That’s absurd. The shape violates what is essential (and not accidental) to the triangle, so it cannot be a triangle. Same could be said for a square circle.
That an accident could be preserved apart from a subject is not a violation of what is essential to the either subject. To be an accident is to not be part of (for or against, really) the essence of a thing.
Still briefing myself on this. But it’s not that the body, blood, soul and divinity of Christ has the accidents of bread and wine, it’s that the accidents are preserved by divine grace in the absence of a subject.