Vertical Extensions on these Walls overlap. This can cause undesirable Joins. Try starting with no overlap and extend the Walls toward one another.
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 26.3.0.0 (26.3.0.0)
Syntax
C#
public static FailureDefinitionId ExtensionsOverlapWarning { get; }