std::is_move_assignable_v false though there is a move assignment operator

Context: I've a class DLXMatrix with some attribute which are vector of some local class called Header. Each Header holds some pointer on some other Header which refer to elements of the same vector (think of a structure which is both a vector and a doubly linked list). As a consequence, I cannot use the default copy constructor and assignment operators since they would point to the original elements and not the copy. Note: I'm making sure that the vector is never resized or moved in the memory.

#include <type_traits>
#include <vector>

class DLXMatrix {
  private:
    struct Header {
        Header *left, *right;
    };
    std::vector<Header> heads;
  public:
    DLXMatrix() = delete;
    explicit DLXMatrix(size_t nb_col);
    DLXMatrix(const DLXMatrix &);
    DLXMatrix &operator=(DLXMatrix other);
    DLXMatrix(DLXMatrix &&) = default;
    DLXMatrix &operator=(DLXMatrix &&other) = default;
    ~DLXMatrix() = default;
};

static_assert(std::is_move_constructible<DLXMatrix>::value);
static_assert(std::is_move_assignable<DLXMatrix>::value);

If I'm not mistaken, though I defined custom copy and assigment operator, the default destructor, move constructor and move assignment copy should work as expected without leaking. Now, I'd like to use std::swap but it refuses to compile because my class is not move assignable:

dlx_matrix.cpp:257:5: error: static_assert failed due to requirement
      'std::is_move_assignable_v<DLX_backtrack::DLXMatrix>'
    static_assert(std::is_move_assignable_v<DLXMatrix>);

So my questions are:

• is this a reasonable way of doing things ?
• why is DLXMatrix not move assignable ?

If it matters I'm compiling with g++ 7.5.0 and clang++ 6.0.0 with standard c++17.

I'm posting an answer following the indication of @IlCapitano.

In the implementation of operator= I needed to do a copy and decided to perform it thanks to a call-by-value. So the prototype was

DLXMatrix &operator=(DLXMatrix other);

instead of the standard

DLXMatrix &operator=(const DLXMatrix &other);

This call-by-value form is not recognized as a proper copy assignment operator by the compiler/library.