This issue has been fixed in Python 3.8 with my work on refactoring Py_Main(). -E and -I command line options are now parsed, before reading PYTHONHASHSEED, and -I imply -E as expected. Extract of the code:

    if (config->isolated > 0) {
        config->user_site_directory = 0;
    }

    if (config->use_environment) {
        err = config_read_env_vars(config);
        if (_Py_INIT_FAILED(err)) {
            return err;
        }
    }

where config_read_env_vars() indirectly reads PYTHONHASHSEED.

I'm not sure if the issue is fixed in Python 3.7 or not. The code in Python 3.7 was in a bad state. It's getting better with Python 3.8 :-)

Note: the overall refactoring work is related to PEP 432 and PEP 587.