Understanding Access Token Validity for Shopify Public Apps

[Hinge123]

I am using the Shopify Admin API and GraphQL API to fetch products and orders. To do this, I generate an access token and store it in my database. I integrated a live store yesterday, generated the access token, and successfully fetched the data. However, the next day, I received an error stating that the "Access token is invalid."

I have read the documentation and learned that access tokens for public apps do not expire and that there is no scenario for refresh tokens. I am confused as to why I am receiving the invalid access token error. Please help me.