I have U2419HC and this issue is drives me nuts. But lowering contrast as you are suggesting has helped! Factory default is 75 contrast, but lowering to 72 has fixed the issue. Any idea what is going on? Also, I have the latest firmware. Really disappointing this new "UltraSharp".

Update: So this contrast trick at 72 is not working anymore and also DELL refused my complains because it's a "feature" of these displays  

Fun fact: My old DELL P2414H has not these issues but newer model from higher range is worse. It seems like i'm done with this company for good.