A thin line imported as an image (two-pixel height in 300 dpi resolution) is not getting displayed while making preview in Designer but is shown in other output formats like Advance Function Print Stream (AFPDS).
EngageOne Designer's PDF Output device is configured to use 72 dpi, therefore the 300 dpi image will be downscaled to 72 dpi. The result of this is that a two-pixel height image will downscale to a fraction of one pixel in 72 dpi so cannot be displayed.
Increasing the height of the image to four pixels will be enough to display when the 300 dpi image is downscaled to 72 dpi. This is an ideal adjustment if the same image resource will be used for other output that is set to 240, 300 or 600 dpi.
UPDATED: November 11, 2019