The best way to avoid this is to build the roundabout by hand.
First, lay out some dirt roads (because they're cheap) in a cross pattern sized to the diameter of your roundabout.
Next, draw the roundabout roads starting from each node of the cross. Use the curved road tool and turn road guidelines on.
Now, leave the dirt roads for bracing. You can now add roads at any point in the segments and they will not distort.
When done, bulldoze the dirt roads you used for bracing.
If you need to modify the roundabout later, put back the bracing roads first.