I have a closed shape consisting of a bunch of joined splines and joined polylines, my problem is when I try join a curved spline to a joined section of straight polylines the curved splines straighten out, then when I pick spline again the splines go curved again but so do the straight polylines.Anyone know how to get them all joined with the splines staying curved and the polylines staying straight?

Do any of these conditions exist?Your pline and spline are not created on the same UCS?
One or the other has been rotated out of it's original plane?
Both entities non-coplanar?BTW...I tested your problem using AutoCAD 2018 and had no problem with connecting a spline to a polyline or vice-versa.  Neither changed form.
John Connor2018-03-05 14:04:45