The validation plugin focusses on the first invalid form field if validation fails, scrolling the page if necessary so the focused field is at the vertical centre of the page (the scrolling is browser behaviour on focus?).
However, for an object picker the actual validated form field is hidden so cannot be focussed, so the page does not scroll if it is the first invalid field. A customisation is required to work around this.
In addition, if an object picker has a quick add button, the error message will be placed between the picker and the button:
It should be after the button so the formatting is maintained and the button stays in its correct position.