Never had an issue with my Neptunes.....liquid Tide, and downy, washed in all temps available at any given time, never overloaded, bleach used on a regular basis, never kept the door open.......and never heard of any mold issues from anyone, until I heard about it on this site about others......
thats not without saying....my sisters 1970 solid tub speed queen, for its entire life was only hooked up to cold water, a variety of detergents and such, don't recall her ever using bleach, lid kept closed at all times, machine was in the basement, and you could smell the odor whenever you walked into her house, in 1986 she got a GE FF, the smell went away, but you couldn't tell her, she would blame the odor on a damp basement, and from the heater...
from 1986 to 1998 I had a frigidaire tall tumbler, normal routine for washing, never kept the door open, and never had a mold or smell issue........
but it must come from a varety of conditions, moist, warm, dark, detergents, softners, water conditions, or even what the machine is constructed from.....
even Whirlpool has a few machines with a grey tub that is actibacterial, to reduce smell and odors......in a machine that would not normally have these issues.....but makes you wonder why the plastic in newer FL don't have something built into the plastic to help eliminate this issue....