Quick rundown on Hardcore Darkness: At light level 0 or at nighttime, things become pitch black instead of just the usual ambient lighting Minecraft has.
The issue? You can't have cool headlight helpfulness at night with MTS!
Their git here

EDIT: Possible theory is that due to how Hardcore Darkness makes entity models dark, it may be darkening the beams as well.

Will get with them to see if they can tell me where to look.

So, they didn't seem to want to help with this issue. I'm closing it on my end due to the fact I've done all I can do, but you can bet your bippy I ain't closing it on theirs till they respond to the ticket.