Are you in a position to try different cables and adapters? Over the years I've seen a lot of sensitivity with the NUCs when it comes to cables and adapters. For example right now I'm working with my DE3815. It's been working perfectly with an HDMI cable on my workbench. I replaced the cable with a new, shorter one I just received, and now the NUC has no display on boot. Switching to VGA or a different HDMI cable works, but I've tested the new cable on several TVs and other NUCs and it seems perfectly fine. I've had similar issues in the past with miniHDMI to HDMI cables.
So my advice is to try alternate cables and adapters.
You can also experiment by entering BIOS and switching the primary/secondary monitor settings.
i test several monitors and several hdmi adapter, all the time the same problem.
but i ve found an solution:
i ve bought an avtive hdmi -> vga adapter (~60€), and connect the digital monitor at the displayport
-> all monitors at all the time are working now (analog/hdmi)
Glad to hear you have a workaround, but I'd still like to see a solution that doesn't require additional hardware.
i think its an BIOS issue,
but the workaround is a good solution.
No criticism of your solution meant...just saying it shouldn't have been necessary for you to buy an additional part.