@arthy The LineCombiner (new name for the LineJoiner) can preserve the arcs if you set the Advanced Parameter: Preserve Lines as Path Segments = Yes.
Most FME writers will stroke the Arc in the path if the format does not support arcs - like shape. I've attached an example workspace (2018.1): LineCombiner.fmw
If you do need to stroke the arcs, then you probably have to use Snapper or Anchor Snapper @egomm suggests. AnchorSnapper will allow you to snap the arcs to the lines (or vise versa) giving you a little more control.
You could try a snapper prior to the linecombiner