Our collection occurred on an overcast day with no direct sunlight. The resultant NDVI image did not show any obvious hotspots.
Did the orthophoto look good too? Some artefacts in orthophotos don't show much in NDVI since it is a normalized index. So, if it is an artefact occuring similarly in all bands (or particularly in Red and NIR bands that are relevant for NDVI), then NDVI might look good.
Anyway, I got better results (not perfect) with Average blending mode instead of Mosaic.
Thanks,