Test Coverage Does NOT Count lines coverred by Selenium Webdriver

Question

This is a flask app with a rather simple pytest setup.

import subprocess
import time
import pytest
from selenium import webdriver
from selenium.webdriver.chrome.options import Options

@pytest.fixture()
def start_flask_app():
    # Start Flask server with coverage tracking so Selenium can interacts with it
    server = subprocess.Popen(['coverage', 'run', '--source', 'server', '-m', 'flask', 'run'])
    time.sleep(3)  # Wait for the server to start
    yield
    server.terminate()  # Stop the server after tests are done

@pytest.fixture()
def driver(start_flask_app):
    options = Options()
    options.add_argument('--headless')
    options.add_argument('--disable-gpu')
    driver = webdriver.Chrome(options=options)
    yield driver
    driver.close()

def test_health_page(driver):
    """
    Test that health page works
    """
    driver.get('http://localhost:5000/health')
    assert 'UP' in driver.page_source

I runs test with pytest --cov=server. Test cases pass. But my health route is not covered in the report.

Any idea why? Is this relevant? I tried to tweak my coveragerc but with no luck.

I am using

pytest==7.4.2
pytest-cov==4.1.0