I seem to recall some discussion by various testers during beta that the linetype scale interpretation (LTSCALE) improvement in 12 may only work with a file Open or file Merge....? So, you might try one of those and if that is the case, you may need to Open/Merge onto a worksheet and place THAT on the layout, rather than placing the dwg directly? Also, to get the LTSCALE correct on export, I believe someone said that your conversion setup needs to refer to a dwg template having the desired LTSCALE value.
From the 12 'New Features Guide':
The LTScale variable used in DWG files will now be correctly interpreted when opening or merging DWG files in ArchiCAD. New line types will be created as needed, regardless of the length of the original line pattern.
PS I just noticed your signature lists 11, not 12, so I'm not sure what the trick is there off the top of my head.