I am having the same issue with a Mac Pro late 2013.

I have daisy chained two U2518D, both with DisplayPort 1.2 and MST enabled. I have also tried DP 1.2/MST Enable in the first U2518D and DP 1.2/MST Disable in the second U2518D. But I only get mirroring the screens, not extending them.

My current setup is the following:
Mac Pro mDP --> DP in #1 U2518D (DP 1.2/MST Enable)
#1 U2518D DP out --> mDP in #2 U2518D (DP 1.2/MST Disable)

But the screens are mirrored instead of being extended. Please, could someone tell me what I am doing wrong?

Thank you very much in advance.