Files not uploading remotely

So I’m able to effectively upload image files to an S3 bucket on AWS using the ‘App Connect S3 Upload’ method as described in this tutorial:

However, when I implement the older upload method (‘Uploading Single File to Your Server’), where the files are first uploaded to a folder of my website, the files do not appear in my bucket. It works fine locally, but doesn’t work when my website is deployed remotely to my EC2 instance. I can see that the files do get uploaded to the remote folder though.

The CORS and bucket policies are correct (because they work for the alternative method), so I’m wondering if this is likely to be a permissions issue with the remote image folder on my website? Has anyone had a similar issue?