Хостинг Хероку работает таким образом, что файлы, создаваемые приложением, временны.
The Heroku filesystem is ephemeral - that means that any changes to the filesystem whilst the dyno is running only last until that dyno is shut down or restarted. Each dyno boots with a clean copy of the filesystem from the most recent deploy. This is similar to how many container based systems, such as Docker, operate.
Используй внешний хостинг для хранения файлов.
https://help.heroku.com/K1PPS2WM/why-are-my-file-u...