python-dotenv not loading .env variable for API_KEY, but other variables load fine when override

I'm working on a FastAPI project where I use python-dotenv to load environment variables from a .env file. Most variables load correctly, but the API_KEY variable is not being picked up by my app, even though it is present in the .env file. Other variables from the same file load as expected.

What I have:

• .env file (in the same directory as my server.py):

OPENAI_API_KEY=sk-xxxx
GOOGLE_API_KEY=AIzaSyxxxx
API_KEY=your-secure-api-key-here
PORT=8000

• Code to load and print env variables:

from dotenv import load_dotenv
from pathlib import Path
import os

env_path = Path(__file__).parent / ".env"
load_dotenv(dotenv_path=env_path)
print("Loaded environment variables from .env:")
if env_path.exists():
    with open(env_path) as f:
        for line in f:
            line = line.strip()
            if line and not line.startswith("#") and "=" in line:
                key = line.split("=", 1)[0]
                print(f"  {key} = {os.getenv(key)}")
else:
    print(f".env file not found at {env_path}")

• When I run the app, all variables print correctly except API_KEY, which is jsdnfasdfiejaklmcasljrlkjjh323jcksdfjk.

What I’ve tried:

• Verified .env is in the correct directory.
• Restarted the server after editing .env.
• Ran echo $API_KEY in my terminal (it is empty).
• If I start the server with a clean environment (e.g., env -u API_KEY python server.py), it works. If I start it normally, the wrong value comes back.
• No export API_KEY=... in my shell config files.

Environment:

• macOS
• Python 3.x
• FastAPI
• python-dotenv

Running with uvicorn server:app --reload

Work around

Used load_dotenv(dotenv_path=env_path, override=True) (Works fine but need to know reason why it happens).