I have an Intel NUC (NUC7i5BNH) that has an Intel Iris Plus 640 chipset for graphics. The NUC provides a USB 3.1 Type C port instead of DisplayPort. To this box I have attached two Dell U2415 monitors
USB-C -> DP adapter (mDP in) -> DP-in Primary Monitor (DP out) -> mDP-in Secondary monitor
The issue is that under Ubuntu Bionic the displays goes black as soon as I switch to MST mode (DP enable on primary, DP disabled on secondary). The thing that surprises me is that setup (primary monitor extended to secondary) works perfectly fine under Debian 9 (Stretch). It also works without an issue under Windows 10 Pro.
I have tried several things from disabling 'workspaces on primary display only' to even upgrading the Kernel. Software has been updated multiple times. I tried Linux Mint LiveCD and even Mint had the same issue. Both the screen would go black.
The displays mirror each other when MST is switched off (i.e. DP 1.2 disabled in both primary and secondary monitor) but that defeats the purpose of having two monitors. There is no change made to cabling in either case.
What am I missing here? Is this a bug in Ubuntu? I am running GNOME.