All of the blocks are 'annotative' which means that when the viewport scale is changed, they will scale automatically (when they are turned on for that scale) so that they remain the same size. This is a convenient feature as it allows you to place objects in model space and display them as the same size regardless of viewport scale. It also has the feature of allowing you to move them in a given scale and so display them in one place for that scale and in a different place in another scale. When you hover over an annotative object the cursor will display the little triangular symbol. At the bottom right of the drawing window you will see the viewport scale and two (when in paperspace) buttons that have a similar triangular symbol. One of the toggles between 'always display annotative objects' and 'display object for current scale only'. When toggled to the second option, the annotative objects that have not had that particular scale added to their properties will not display. That's what is going on with your drawing.
