The image splines2012.png is a screenshot of ACCEL1C11.dxf opened in ACAD 2012, I opened the file with CorelDraw (version 12 I think) and the result is the same as in ACAD 2012
The image splines2010-partial.png is a ACAD2010 zoom on the parts that differ with ACAD2010 & LibreCAD
The file ACCEL12to00.dxf is the original dxf opened with ACAD2012 & saved as dxf v2000
Seems that the 2012 version performs some checks on the splines and correct it, like:
if (startPoint == endPoint) spline.isClosed
I tested with this patch
+ if( sd.flags == 1 ) sd.flags = 8; // possible typo 11 != 1
and it works well.
I made some more test:
In librecad draw splines one closed & other open
save as test1.dxf
open with librecad with this patch works
open with autocad2012 all splines are open
in autocad2012 draw a closed spline using the method "control vertex"
draw another spline using the method "fit points"
save as test2.dxf
open with librecad: works well
open with an text editor the code 70 are "2059" & "3339"
I do not understand :-(
Another think about ACCEL1C11.dxf:
from DXF reference "code 210 = Normal vector (omitted if the spline is nonplanar)"
In ACCEL1C11.dxf is present so the flag "8 = Planar" should be placed