Line disappears if rotated 90 degrees in either direction within a container. The line is listed in the property editor but when selected it does not appear in the container. The line component can be rotated without issue outside of the container.
Right, that is a known problem with making a straight vertical or horizontal line. In doesn’t matter which container it is inside.
In this case, you should make a rectangle with either a height or width of 1 pixel that has a black fill and no stroke. That will rotate no problem.
This has been fixed for 7.4.3