The localizers showing up as rectangles are by design. A rectangle or trapezoid is shown when the slice is not orthogonal to the localizer. When orthogonal, it's shown as a line, as the lines of the rectangle would pile up on each other. We've use this approach as it's more accurate.
When you see a single line on other viewers, they just use a line in the center of the rectangle, a diagonal, longest projected side, etc. This may not be correct and may result in the same type of error