In all drawing program, we can change the size of an object independently or locked for with and height, even in arcmap. But, in Pro, the layout is always in lock so changing one is reflected to the other one. I try to find a lock somewhere without success.
The only way I find is via insert and manually drawing a new box to reshape the map, in my layout.
So, if it's not because I haven't found this feature.
In Map Frame, could it be possible, to simply add a lock to choose or not to link the width and the height.