Whenever we make changes to existing documents, the API doesn't display these changes and it usually takes a few hours for them to appear. For one of our documents, we changed an image and it has been 24 hours that the new image URL hasn't appeared in the API response.
We even tried the Prismic API browser and it has the same issue so it's nothing to do with caching on our end.
So I made a test clone of your repo to debug this and i was able to update the image write away.
This would suggest maybe it’s a application side issue.
How are you passing the master ref in your project?