I didn't expect first grade math to confuse someone this much. Consider having yourself tested if you haven't already, unless you're still in first grade, in which case you should probably be doing your homework instead of writing insightful commentary online, you'll need the extra credits, trust me.
CC reduction effects of specific classes are completely irrelevant because I was addressing OP's claim that 23 seconds of cooldown on psychic scream somehow means it can't trigger diminishing returns from itself which is false because 8 + 15-20 = how much? That's right, 23+, which means you can trigger diminishing returns that way if you wait for the full duration and then fear again as soon as possible. Another thing is OP never mentioned this happening to any specific classes, he just assumed this is some kind of general bug because he thinks DR starts ticking when the CC lands as opposed to when it ends, which again makes your comment pointless. Third, 30% fear reduction gives us a fear duration of 5.6 and 5.6 + 18-20 is still 23.6-25.6, which means you can still trigger diminishing returns with just psychic scream if you're unlucky with DR timers and fear exactly on cd, which once again makes me wonder what point you were even trying to make here.