Filenames changing after exporting from photos.

Every photo uploaded to iCloud Photo Library is given a long, randomized filename. The original filename, however, still remains as part of the image metadata. You will see the long, randomized filename or the original filename depending on how you export, download, or which device you email the file from. It's nothing to worry about. This is done to ensure every image has a unique filename in your iCloud Photo Library no matter where it came from. If you have more than one device, each of those devices start out with their first image being named IMG_0001.JPG and then both are going to upload that image with the same filename. Or on your Mac if you manually give an image a name, you don't want to keep track of whether you've used a filename already.

If you email an edited or unedited image from your Mac, the recipient will see the original filename. If you email the same edited image from your iPhone, the filename will be FullSizeRender.jpg. The unedited image emailed from the iPhone will have the long filename. If you are emailing a recently taken photo from your iPhone, the short filename, e.g., IMG_0123.JPG, will be used because it's still on-device and did not have to be retrieved from the cloud.