AccessToken Different from that required by the backend

We are in the last step of the atutification flow. We are a group of 3 completely new and inexperienced developers. My frontend partner is working on React.js with Next.js. A friend and I are on the backend and we are working with python and django rest framework. The problem that is driving us crazy is that when the user enters our page the access token that the frontend provides is different than the one that actually works according to what we tested through the postman. But if the frontend tests that token that doesn’t work for the backend, it tests it in /userinfo (if it works). Then we are in a limbo where we don’t know what to do. We are waiting for your answer with intentions to help us or to calm our doubts.