We are aware of that (you actually reported it already here: Create a proper technical drawing from a 3d model - #18 by Toni_Osterlund). I’ll let you know when we fix this.
This print preview doesn’t seem to be consistent with the printing color of the beams, which is set to “by Layer”, and has the black color assigned. So I think the only problem here is the Print preview page, because the beams print out in the right color.
I’m surprised you can print the beams if you are using the Technical display mode in that detail (because of the first issue listed here). I cannot reproduce it.