Steps to reproduce:
Create two objects
Select object A
Move it upwards a bit by pressing the shift and arrow key on your keyboard
Drag and move object B (don't (de)select anything in between, just click and drag right away)
What happens:
When object B is dragged a single pixel in any direction with the mouse, it immediately offsets by that far in the same direction as object A was moved with the keyboard just before. That shouldn't happen. Object B should stay where the mouse cursor is when it is dragged and moved somewhere.
I guess Designer internally remembers some kind of offset from the other input method (keyboard) and erroneously applies it to the other object (mouse input).
Affinity Designer 1.7.1 on Windows 10.