Hello,
It seems that illustrator do not handle arcs in the DXF, they are converted to lines (maybe there is a setting to define the number of point used for the conversion in ai)
In CB, use Polyline/remove arcs (on your drawing I use tolerance =0.05), then export to DXF
The file without arcs has no problem and can be filled in ai without joining anything.
I attach a converted file.
++
David