Why is compiler mis-stating namespace?

Doing some C++ development after a good four years in Python, it appears I'm a little rusty.

I have three files in my very cut-down example (the actual code is much larger but this example still exhibits the problem). They are the code for the main file, and the code and header for a separate component that it will use:

// xx.cpp:
#include "xx_store.h"
int main() {
    auto store = XX::XXStore();
    return 0;
}

// xx_store.cpp:
#include "xx_store.h"
namespace XX {
    XXStore::XXStore() : m_map(XXEntries()) { }
    XXStore::~XXStore() { }
}

// xx_store.h:
#include <map>
#include <string>
namespace XX {
    using XXEntry = struct {
        std::string m_value;
        bool m_found;
        uint32_t m_age_ms;
    };
    using XXEntries = std::map<std::string, XXEntry>;

    class XXStore {               // WHERE THE ERROR IS REPORTED.
    public:
        XXStore();
        ~XXStore();
    private:
        XXEntries m_map;          // THE APPARENT CAUSE.
    };
}

Now, when I compile that with g++ --std=c++11 -o xx xx.cpp xx_store.cpp, I get two warnings, one for each of the code files, but both referring to the same place in the header file:

xx_store.h:12:11: warning:
    ‘XX::XXStore’ has a field ‘XX::XXStore::m_map’ whose type uses
        the anonymous namespace [-Wsubobject-linkage]
        12 |     class XXStore {
           |           ^~~~~~~

Now I know from past experience that anonymous namespaces are per translation unit so it's unwise to do that in a header file that may be included by multiple code files.

But I can't see how the type of the m_map variable (which is XXEntries) is actually in the anonymous namespace. It's clearly inside the namespace XX { ... } area in the header file, as is the XXEntry type it depends upon. In fact, the warning even states that it's within the XX:: part of the namespace.

Can anyone suggest why this warning may be happening? I know I can disable the warning but I'd rather not go down that path until I'm sure it's a false positive.

The environment is Ubuntu 20.04 running g++ 9.4.0.