[BUG]: Unable to convert function return value to a Python type

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Problem description

Consider a module mymodule. From it, I am returning a reference to a member as follows

const GooseFEM::Vector& vector() const { return m_vector; }

with corresponding Python API

PYBIND11_MODULE(mymodule, m)
{
    py::class_<Myclass> cls(m, "Myclass");
    ...
    cls.def("vector", &Myclass::vector, "vector");
}

Now GooseFEM has its own (pybind11) Python API, so this works brilliantly.

Accept... Since a week or so, the package of GooseFEM (and other libraries, just making an example here) shipped from conda-forge is no longer playing nicely with my locally compiled library on Linux (macOS and Windows work fine).

I have no idea what is going on, but have been able to get a minimal reproducer: https://github.com/tdegeus/test_pybind (which include the failing CI on Linux, and passing CI on macOS and Windows).

Reproducible example code

See full working example, including CI: https://github.com/tdegeus/test_pybind

#include <pybind11/pybind11.h>
#include <pybind11/stl.h>
#include <GooseFEM/GooseFEM.h>

namespace py = pybind11;

class Myclass
{
public:
    Myclass() = default;

    Myclass(size_t n) {
        m_mesh = GooseFEM::Mesh::Quad4::Regular(n, n);
        m_vector = GooseFEM::Vector(m_mesh.conn(), m_mesh.dofs());
    }

    const GooseFEM::Vector& vector() const
    {
        return m_vector;
    }

private:
    GooseFEM::Mesh::Quad4::Regular m_mesh;
    GooseFEM::Vector m_vector;
};

PYBIND11_MODULE(mymodule, m)
{
    m.doc() = "Foo";
    py::class_<Myclass> cls(m, "Myclass");
    cls.def(py::init<size_t>(), "Myclass", py::arg("n"));
    cls.def("vector", &Myclass::vector, "vector");
}

and Python code

import GooseFEM
import mymodule

a = mymodule.Myclass(3)
v = a.vector()
print(v)

Start with the linked test_pybind minimal reproducer and its failing Linux CI, then inspect the Myclass::vector binding and GooseFEM conversion shown in the issue. Compare the Linux result with macOS and Windows; done means the reproducer returns the vector successfully on Linux or the incompatibility is isolated and documented.