LINQKit Leftjoin/GroupJoin : The query cannot be translated

Question

I'm using LINQKit in a query with LeftJoin in it :

return _context.User
            .LeftJoin(
                _context.JournalAcces,
                u => u.Login,
                j => j.LoginUtilisateur,
                (u, j) => new { U = u, J = j })
            .AsExpandable()
            .Select(x => new User
            {
                Login = x.U.Login,
                Date = x.J.Date,
                IsUsed = IsUsed().Invoke(x.U),
            })
            .ToList();

I get this error message : the LINQ expression could not be translated But if I replace LeftJoin by Join, it works.

I also tried adding LINQKit configuration on DbContextOptions level :

builder
    .UseNpgsql(connectionString)
    .WithExpressionExpanding();

and deleted the .AsExpandable() from the query, but I got the same error :

return _context.User
                .LeftJoin(
                    _context.JournalAcces,
                    u => u.Login,
                    j => j.LoginUtilisateur,
                    (u, j) => new { U = u, J = j })
                .Select(x => new User
                {
                    Login = x.U.Login,
                    Date = x.J.Date,
                    IsUsed = IsUsed().Invoke(x.U),
                })
                .ToList();

Is this a bug in LINQKit or a limitation ? because I couldn't find any reference for it in the documentation nor in the open issues.

For reference, IsUsed is defined like this :

public static Expression<Func<User, bool>> IsUsed()
    {
        return x => x.Employement == "Employed";
    }