Imaging subsystem artifacts. When looking very near the sun, all kinds of undesirable effects like diffraction, scattering and internal reflections occur.
Take for example Cassini. Its wide angle camera shares the same optics as the Voyagers WAC and it's notorious for scattered light artifacts. Here's a comparison slideshow
between what and how Cassini's cameras see things at high phase angles. Note the scattered and colorful "sunbeams" in the wide angle view.
As with Cassini, Voyager narrow angle cameras had better performance at very high phase angles, but they too had a limit. At the time that famous image was taken, Earth was located pretty close to the Sun so even the narrow angle suffered some diffracted sunlight.
Wide-angle cameras are generally meant to provide context for narrow-angle shots and as such they aren't the
workhorse camera on a spacecraft. Accordingly, less stringent design requirements are placed on them, such as wider point spread functions are allowed, narrower spectral sensitivity, less sun shielding (in this case, probably due to mass constraints) etc.