The Cloudinary Bulk Extension requires some adjustments to function properly. The Transformation URL is not straightforward and requires modification. Here's an example for your reference.
This is an example of the transformation URL from Cloudinary.
https://res.cloudinary.com/xxxxxxxxx/image/upload/cld-sample.jpg
When saving it to the bulk extension setting, you have to modify it to:
https://res.cloudinary.com/xxxxxxxxx/image/upload/sample.png