I completely understand that you would like to be able to get a rectangle in the simplest way possible. But the operator is a bit more complicated. It can not only create rectangles but also complex shapes with straight edges. So you need to indicate which vertex is a corner. You have a choice between a corner topology (only two edges at the vertices of the island), and vertices that are pins. In your case, this island is triangulated, so it does not have corner vertices. Your only choice is to specify the corner vertices using the pinning method.

@Ayami , https://zenmastersteam.github.io/Zen-UV/latest/transform/#rectify