Check the setting of the "Move Tool Aspect Constrain" option in Affinity Designer > Preferences > Tools. There are 3 possible settings:
Automatic (based on selection) -- constrains only objects that have a 'natural' aspect ratio (like Image layers)
Constrain by default -- constrains unless the Shift key is held down
Do not constrain by default -- constrains only if the Shift key is held down
So basically, if the aspect ratio is not being constrained when you drag a corner with the shift key held down, try it without holding down the shift key, or choose the option above that you prefer.
