1) What happened

Marshalling for field of struct with type UnityEngine.Vector3[] is not
supported

2) How can we reproduce it using the example you attached

this c# code:

Vector3[] Corners = Path.corners;
FinalPos = Corners[Corners.Length - 1];

translates into this as3 code:

var $corners: CLIObjectArray = this.GOAPActionCheckLostEnemy$Path$.corners;
this.GOAPActionCheckLostEnemy$FinalPos$.cil2as::Assign($corners.elements[$corners.Length
- 1] as Vector3);

and throws error:

Error: Marshalling for field of struct with type UnityEngine.Vector3[]
is not supported (NavMeshPath.m_corners).
at UnityEngine::NavMeshPath/MarshalTo()
at com.unity::UnityNative$/Ext_MarshallTo()
at
com.unity::UnityNative$/NavMesh_CUSTOM_INTERNAL_CALL_CalculatePathInternal()
at
UnityEngine::NavMesh$/NavMesh_INTERNAL_CALL_CalculatePathInternal_Vector3Ref_Vector3Ref_Int32_NavMeshPath()
at
UnityEngine::NavMesh$/NavMesh_CalculatePathInternal_Vector3_Vector3_Int32_NavMeshPath()
at
UnityEngine::NavMesh$/NavMesh_CalculatePath_Vector3_Vector3_Int32_NavMeshPath()
at
global::GameZoneBase/GameZoneBase_GetCoverAgainst_AgentHuman_AgentHuman_Vector3_Single()[D:\Dev\4.0\ShadowGun-Flash\Temp\StagingArea\Data\ConvertedDotNetCode\global\GameZoneBase.as:278]