The problem with Shapes is: you cannot resize the shape from the Generator. When you go to the Video Inspector > Scale, you are resizing the entire width of the viewer and the Rectangle gets squeezed into the resized/scaled view. As soon as you move the rectangle/shape left or right - it's literally disappearing off the edge of the "screen" (shrunken scaled view).
Use this instead:
http://fcpxtemplates.com/ngtopics/Playhead.zip
It has a nice (optional - but on by default) "head" on it. It has a Line that can be resized (width and height) and a Color parameter to choose the color. It includes an OnScreen Control to ease positioning. If you need a Drop Shadow - there's an Effect for that - just drop it on the generator and adjust to taste.
[PS - OSC position values are decimal "percentages" of the size of the viewer. The range is -0.5 to 0.5 horizontally and vertically regardless of the "aspect" of the dimension. If you dial down the disclosure triangle, the ranges for X and Y are 0.0 to 1.0.]

Here are the published parameters:

If you need help installing:
https://fcpxtemplates.com/installing-plugins-for-fcpx/