Add constraints to shapes inside union

Yes, you need to wrap the union in a frame, then you will be able to assign constraints to elements inside of it. Constraints can only be set on objects inside frames, and groups/boolean operation objects are ignored, the objects inside of them will stick to the outer frame.

This bug still exists three years later. This is the (unfortunately annoying) workaround. Thank you!

Not sure why Figma’s interface would pretend to let us change constraints that are then ignored anyway 😦

Not a bug. Groups work this way too, and you can not only change constraints but also colors and other properties, however the group itself doesn’t have any of such properties, it changes the objects inside.