When the DialogManager is first instantiated, it finds a random parent canvas using FindObjectOfType<Canvas>. Unfortunately for us, we have some canvases used as 3D UIs which have been scaled. Randomly one of these gets chosen as the DialogManager's parent, and the dialogs end up massively out of scale.
I'm sure you'll know what a good solution is for this. My suggestion would be to search for a Material UI Scaler and use the canvas/transform associated with it.