Thank you kindly, Grumly57 for a patch that solves:

When a sit position is modified by llSitTarget and the 
sim is restarted, the object sit position is lost until 
llSitTarget is applied again.

OpenSim/Region/ScriptEngine/Shared/Api/Implementation/LSL_Api.cs

@@ -6049,6 +6049,7 @@ namespace OpenSim.Region.ScriptEngine.Shared.Api
 
             m_host.SitTargetPosition = new Vector3((float)offset.x, (float)offset.y, (float)offset.z);
             m_host.SitTargetOrientation = Rot2Quaternion(rot);
+            m_host.ParentGroup.HasGroupChanged = true;
         }
 
         public LSL_String llAvatarOnSitTarget()