YAML::LoadFile changes a data file

I'm trying to use yaml-cpp to serialize(and de-serialize) "scene" data for my game engine. The problem is, YAML::LoadFile seems to change a scene data file when loading multiple files.

Let's say I have 2 files, A.yaml and B.yaml.

A.yaml:

-entity1
-entity2
-entity3

B.yaml:

-entity1
-entity2
-entity3

After adding 2 more entities and saving it, I can see that B have chaged properly.

B.yaml after chage:

-entity1
-entity2
-entity3
-entity4
-entity5

But if I load A and load B again in a same runtime, the file B returns to it's original state! (B.yaml at first). If I save B and restart my engine, then loading files multiple time has no effect on the file B.

Here's my code for loading and saving.

class scene_data {
public:
    void save(const std::filesystem::path &) const;
    void load(const std::filesystem::path &);
    std::vector<entity_data> entities() const;
    void add_entity();
    void delete_entity(const entity_data &);

private:
    std::vector<entity_data> _entities;
};

void scene_data::save(const std::filesystem::path &path) const {
    YAML::Emitter em;
    em << YAML::BeginMap << YAML::Key << "entities";
    em << YAML::BeginSeq;
    for (const auto &e: _entities) {
        e.serialize(em);//Each entity write themselves in emitter
    }
    em << YAML::EndSeq;
    em << YAML::EndMap;

    std::ofstream filepath(path);
    filepath << em.c_str();//I've heard that the ofstream doesn't need to be closed manually.
}

void scene_data::load(const std::filesystem::path &path) {
    auto root = YAML::LoadFile(path.string());

    if (auto ett_node = root["entities"]) {
        std::vector<entity_data> new_entities;
        for (const auto &ett: ett_node) {
            new_entities.push_back(entity_data(ett));//entity_data constructor takes yaml node
        }
        _entities = new_entities;
    }
}

Would there be some rules on using YAML::LoadFile?