It's a little difficult to see in your gif because sometimes the bottom item edge is covered by the tooltip but it looks like the snapping happens when you cross the item start point in the dragged item (the little triangles in the horizontal item edges).
Can this be the case ?
If so, snapping to this point happens here too even with snap disabled and can't indeed be turned off it seems (except with the 'relative edge edit ignoring snap' mouse modifier as you say).
