When a symbol is not aligned to pixel grid, the origin is not aligned to an intersection of the grid. Thus it becomes very difficult to create pixel perfect symbols. Paths that are snapped to the pixel grid are then redisplayed offsetting from the origin, causing bleeding in the isolation mode.
This makes symbols essentially unusable as it becomes impossible to align objects scaled down to look good at the pixel level because such objects do not have easy to work with dimensions. With original objects it's somewhat reasonable to correctly align them numerically.
There is absolutely no reason I can think of why this should be the case and it renders using symbols virtually impossible. This is a bug unless there is a setting to fix it. Essentially the pixel grid is not aligned to the ruler.