As documented in the monit deck,
The output from your check program script is displayed in the Monit UI. In the dashboard, only the first line is displayed.
This is not correct. The http cervlet is writing the entire output to the ui and relying on css to hide what gets written beyond the cell width. If the first line of output is rather short, the dashboard displays the first line + whatever additional output can fit in the cell.
Not sure if the documentation needs to be updated, or if this is a bug. Displaying the first line of output could be quite useful.