The recommendation while connecting monitors/TVs to your NUC is to use straight connections, meaning not to use adapters.
You can test your system using different cables, testing with a different hub or different displays.
I would also like to provide you with this link, in the page 22/23 you can find the supported antested configurations to achieve dual display with this unit.
I look forward to your outcome with this.
If your MST hub does not contain an active DVI converter this will probably not work. The simple passive DP-DVI converters only do level shifting and rely on the DP graphics hardware to detect this and switch protocols from DP to DVI (or HDMI). I don't think this works with MST, as the port needs to remain in DP mode to do multi-stream.
Thank you for your feedback regarding the use of converters.