Vector diffusion in corners

[VaitkusM]

While attempting to smooth out vector and line fields (actually Hopf differentials of a tessellated Bézier surface) with the vertex-based connection Laplacian, I repeatedly found outlying values in boundary corners - as can be seen on these pictures, where Q (in red) is the original line field, while Q (smoothed) (in blue) is the result of 1 step of implicit smoothing with the usual h^2 timestep:

As far as I an tell, I weight everything correctly with the Mass matrix.
It happens whether or not the corner has a single "flapping" triangle.
I have no boundary conditions (so I guess I implicitly prescribe zero Neumann).

What could be the cause of this phenomenon?

Edit: Tried switching lumped mass matrix to Galerkin, weak Laplacian to strong form, as well as various timesteps (smaller/bigger than h^2, and also keeping it to unity) - the corners (and to a lesser extent boundaries) always stand out...

Edit 2: Also tried the uniform weights in place of cotans with and without degree-based mass - the issue remains.
I suspect the problem might be the alignment of the vertex tangent basis vectors: they are everywhere aligned with an outgoing halfedge, except at such corners.

[VaitkusM]

The source of the problem might be the scaling of corner scaled angles at boundary vertices in the IntrinsicGeometryInterface class, which (intrinsically) flattens out the corners:

geometry-central/src/surface/intrinsic_geometry_interface.cpp

Lines 147 to 163 in bafa202

	// Corner scaled angles
	void IntrinsicGeometryInterface::computeCornerScaledAngles() {
	cornerAnglesQ.ensureHave();
	vertexAngleSumsQ.ensureHave();
	cornerScaledAngles = CornerData<double>(mesh);
	for (Corner c : mesh.corners()) {
	if (c.vertex().isBoundary()) {
	double s = PI / vertexAngleSums[c.vertex()];
	cornerScaledAngles[c] = s * cornerAngles[c];
	} else {
	double s = 2.0 * PI / vertexAngleSums[c.vertex()];
	cornerScaledAngles[c] = s * cornerAngles[c];
	}
	}
	}

Setting the scaling for boundary vertices to 1 seems to fix the vertex tangent basis, as well as the connection Laplacian at corners.