Right now, it is not possible to make a square frame that resizes via parent frame. It always distorts into a rectangle.
Even with “constrain proportions” toggle selected, it does not constrain the proportions. To make matters even worse, the toggle only “works” when changing the numeric fields of height/width, which then breaks snap-to-pixel-grid. But then when holding shift and dragging a corner of the frame, it constrains proportions and maintains the snap-to-pixel grid functionality.
I believe a toggle/option in the constraints panel (example: width = fill container, height = maintain proportions) could do the trick.