Directus images disappear after a while

I created a Next.js project that is deployed on Vercel and uses a MySQL database. I then deployed a Directus instance on Heroku that is tied to that same database. In my Next.js project I want to fetch and render images that I uploaded to Directus. At first this works, but after a while all the images disappear in the Directus media library. The folders and references to the images are still there, but I don't see the pictures anymore, I get to see a JPG logo instead. When I try to fetch the images I get a 502 "Bad Gateway" error. I don't know what causes the images to disappear and how to fix this.