Heroku Node js file changes are not saving

I have simple discord bot with leveling system runing on node.js , all user stats are saving into .json file.

Everything looks fine, but when I'm trying to open file via Heroku's bash cat it's showing me state of file when it was originally pushed.

Also, I'm facing issue when after some time user's stats are being deleted, I'm thinking that because file (some how) being wiped back to original state.