"Blotch problem" on mju720SW images

I’m writing to seek help or advise (from anyone) to resolve “blotching problems” on images taken using mju720SW. I noticed that pictures taken at low lights (w/flash) often exhibited blotching. Here’s my observation:
- Blotches only occurred when pictures were taken at low light w/flash. Maybe during daylight, the blotches do exist, but not very visible.
- Blotches occurred regardless of the ISO used (i.e. I’ve seen blotches with ISO80, and up to ISO800).
- Location of blotches is pretty much random.
Note: No issue with camera lens (lens is clean).

Not sure whether this is optics related or image processing errors.

Anyone have seen this before?
Any advice?