Declined comment flags are supposed to be visible on the Flagging Summary page. However, this doesn't seem to work in all cases. It seems as if there are cases when the declined comment flag count is incremented, but there's no user-visible history for it.
Please note that it's not just the reason that's missing, even though the referenced post says there should be one. The real issue is that there's nothing on the Flag Summary page at all referencing the declination, other than the flag count.
Without knowing the internals, it seems likely this is related to deleted questions or comments. However, if the related question/answer or comment is deleted, shouldn't the flag count be decremented in the same way that reputation is adjusted when posts are deleted?