Odd image cropping behaviour

I’m currently experiencing some really funny behaviour with the WordPress custom image cropping tool.

There is an existing image on the site which I’ve tried custom cropping using the ‘edit image’ feature. When custom cropping the preview comes up with the crop that I selected, upon saving the image, the saved crop then appears as a scaled down version of the original image.

To test, I then uploaded a new image, this was cropped successfully.

Once this was done, the previously mentioned image which was cropped unsuccessfully I deleted, confirming it’s removal by checking for any originals by checking through my ‘uploads’ folder using FTP. I then uploaded the same image, using a different image name this time, only for the same problem to happen again.

The unsuccessfully cropped image is to the same ratio as the successfully cropped image. In my functions I’ve made sure that image thumbnails have been activated and specified a size, which I’ve also set in my WordPress settings screen.

I’m currently at a loss to understand why this is happening.