(Pixelmator Pro 1.0.9 on macOS 10.13.4 on MacBook (Retina, 12-inch, Early 2015))
The Constrain proportions checkbox is ignored when I have united shapes.
Create a new file.
Add two rectangles.
Select both rectangles and Unite them.
Select one rectangle, ensure that Constrain proportions is checked and change its width.
Notice that changing the width does not change the height.
Is something not working like it should? Let us know.