This isn't the only place where I have seen this issue; it exists in multiple generations of the NUC (though affecting USB-C only started in this generation, of course). I suggest that you go into the Windows Power Options applet, select Edit Plan Settings for your chosen plan and then set Turn off the display to Never. I have also seen issues with sleep in general, and avoid this by also setting Put the computer to sleep to Never. Yes, it wastes a little power, but I avoid these frequent reboots and "hung" monitors...
Please try the suggestion Scott mentioned above, and share your results. I am really interested in that result.
You can also try Thunderbolt™ 3 Firmware Update Tool with version 18 which you can find here; Download Thunderbolt™ 3 Firmware Update Tool. Find the Update Guide here; https://downloadmirror.intel.com/26141/eng/Thunderbolt-Firmware-Update-Guide.pdf
I have the same monitor, LG 27UD88-W, and it goes to sleep from the internal power saving function in the monitor settings which is enabled by default.
I believe the default was 4h, and after 4h the monitor just goes to sleep, even if you are in the middle of doing something.
What I did was to disable this in the monitor settings.