Many times a problem that occurs when printing a map or exporting to the vector exporters EMF, PDF, AI, EPS, or PNG or to the raster exporters JPEG, GIF, TIFF and BMP at higher resolutions do not occur when exporting to JPEG, GIF, TIFF and BMP at lower resolutions. This is because ArcMap takes a shortcut, if possible, when exporting to JPEG, GIF, TIFF and BMP at lower resolutions, depending on the total image size requested based on the page size and output resolution or DPI.
If the total output image size is the same size or smaller than the image of the map currently being displayed in the ArcMap Layout view interface, the latter is reused for the output. If the total output image size is larger, then the output goes through the normal processing procedure, which can occasionally produce different results.
The conclusion is that when diagnosing output issues, reporting that an issue does not occur when exporting to JPEG at 100dpi, for example, is not helpful diagnostic information.