Bug in SquareTissue if deformation is not plan

[epernod]

Simply change the gravity as gravity="0 -10 -3" and the scene is really unstable

<?xml version="1.0" ?>
<!--Test case 1: Upper border is fixed and only heavy gravity is applied -->

<Node name="root" dt="0.01" showBoundingTree="0" gravity="0 -10 -3">
	<Node name="RequiredPlugins">
		<RequiredPlugin name="Sofa.Component.Collision.Detection.Algorithm"/> <!-- Needed to use components [BVHNarrowPhase, BruteForceBroadPhase, DefaultPipeline] -->
		<RequiredPlugin name="Sofa.Component.Collision.Detection.Intersection"/> <!-- Needed to use components [MinProximityIntersection] -->
		<RequiredPlugin name="Sofa.Component.Collision.Geometry"/> <!-- Needed to use components [TriangleCollisionModel] -->
		<RequiredPlugin name="Sofa.Component.Collision.Response.Contact"/> <!-- Needed to use components [DefaultContactManager] -->
		<RequiredPlugin name="Sofa.Component.Constraint.Projective"/> <!-- Needed to use components [FixedConstraint] -->
		<RequiredPlugin name="Sofa.Component.Engine.Select"/> <!-- Needed to use components [BoxROI] -->
		<RequiredPlugin name="Sofa.Component.IO.Mesh"/> <!-- Needed to use components [MeshGmshLoader] -->
		<RequiredPlugin name="Sofa.Component.LinearSolver.Iterative"/> <!-- Needed to use components [CGLinearSolver] -->
		<RequiredPlugin name="Sofa.Component.Mapping.Linear"/> <!-- Needed to use components [IdentityMapping] -->
		<RequiredPlugin name="Sofa.Component.Mass"/> <!-- Needed to use components [DiagonalMass] -->
		<RequiredPlugin name="Sofa.Component.ODESolver.Backward"/> <!-- Needed to use components [EulerImplicitSolver] -->
		<RequiredPlugin name="Sofa.Component.SceneUtility"/> <!-- Needed to use components [InfoComponent] -->
		<RequiredPlugin name="Sofa.Component.SolidMechanics.FEM.Elastic"/> <!-- Needed to use components [TriangularFEMForceField] -->
		<RequiredPlugin name="Sofa.Component.SolidMechanics.Spring"/> <!-- Needed to use components [TriangularBendingSprings] -->
		<RequiredPlugin name="Sofa.Component.StateContainer"/> <!-- Needed to use components [MechanicalObject] -->
		<RequiredPlugin name="Sofa.Component.Topology.Container.Dynamic"/> <!-- Needed to use components [TriangleSetGeometryAlgorithms, TriangleSetTopologyContainer, TriangleSetTopologyModifier] -->
		<RequiredPlugin name="Sofa.Component.Visual"/> <!-- Needed to use components [VisualStyle] -->
		<RequiredPlugin name="Sofa.GL.Component.Rendering3D"/> <!-- Needed to use components [OglModel] -->
		<RequiredPlugin name="tearing"/>
	</Node>
	
    <VisualStyle displayFlags="showVisual showBehaviorModels" />

    <DefaultAnimationLoop />
    <DefaultPipeline verbose="0" />
    <BruteForceBroadPhase/>
    <BVHNarrowPhase/>
    <DefaultContactManager response="PenalityContactForceField" />
    <MinProximityIntersection name="Proximity" alarmDistance="0.8" contactDistance="0.5" />
    
    <Node name="SquareGravity_FEM">
        <EulerImplicitSolver name="cg_odesolver" printLog="false"  rayleighStiffness="0.1" rayleighMass="0.1" />
        <CGLinearSolver iterations="25" name="linear solver" tolerance="1.0e-9" threshold="1.0e-9" />
        <MeshGmshLoader name="meshLoader" filename="mesh/square3.msh" scale="10" createSubelements="true" />
        
        <TriangleSetTopologyContainer  name="Container" src="@meshLoader" fileTopology="" tags=" " />
        <TriangleSetTopologyModifier   name="Modifier" />
        <TriangleSetGeometryAlgorithms name="GeomAlgo"   template="Vec3d" />
        
        <MechanicalObject name="Mo"/>
        <DiagonalMass massDensity="10.0" />
        
        <BoxROI name="FixedSide" box="-0.1 9.9 0.1  10.1 10.1 -0.1" drawBoxes="1" />
        <FixedConstraint name="FixedConstraint" indices="@FixedSide.indices" />

        <TriangularFEMForceField name="FEM" youngModulus="300" poissonRatio="0.3" method="large" computePrincipalStress="1"/>
        <TriangularBendingSprings name="FEM-Bend" stiffness="400" />
        <PlaneForceField normal="0 1 0" d="-20" stiffness="100000" showPlane="1" showPlaneSize="20"/>
        
        <TriangleCollisionModel />
        
        <TearingEngine name="TearingEngine" input_position="@Mo.position" stressThreshold="10.0" 
            step="30" nbFractureMax="40" showFracturePath="1"/>
			
        <Node >
            <OglModel name="Visual" color="red" />
            <IdentityMapping input="@.." output="@Visual" />
        </Node>
    </Node>
    
</Node>