I spoke to both Apple and Dell about this issue. I could not get a fix from either company. I purchased two of these monitors with a 2018 mac mini and both do the same thing. I also tested with a 2018 Macbook Pro. MacOS thinks these monitors are TVs, which is why it's sending out a YPbPr signal to the displays and not an RGB signal. When the mac goes to sleep, it will wake up to a purple screen, since the Dell monitor switched to RGB mode.
I spoke to both Apple and Dell about this issue. I could not get a fix from either company. I purchased two of these monitors with a 2018 mac mini and both do the same thing. I also tested with a 2018 Macbook Pro. MacOS thinks these monitors are TVs, which is why it's sending out a YPbPr signal to the displays and not an RGB signal. When the mac goes to sleep, it will wake up to a purple screen, since the Dell monitor switched to RGB mode.