Unfortunately, Dell does not test/validate this monitor model on Apple hardware, ports, and software. Because of this, Dell does not have many troubleshooting steps to offer. You could post on the Apple Discussions Forum and see what they say. Some apple users have mentioned that using a thunderbolt to DP adapter has had better results.
apple thunderbolt out --> thunderbolt to DP adapter --> DP to DP cable --> Dell monitor DP in