Image Quality setting has no effect

Problem/Motivation

I don't know what's wrong with my installation but regardless of what number I put in the "image quality" setting at /admin/config/media/webp/settings the image and the filesize is always identical.

I'm not seeing an existing issue here for this so I'm wondering if it's just me, but truly I can't see anything particular in my installation which would cause this, especially without any kind of error message.

Steps to reproduce

1. Have ImageMagick installed and working, set up an image style with a crop and a resize followed by the WebP conversion
2. Go to /admin/config/media/webp/settings and set the image quality to 100
3. Go to /admin/config/media/image-toolkit and set the image quality to 100
4. Flush the image styles
5. Download a processed image
6. Go to /admin/config/media/image-toolkit and set the image quality to 50
7. Flush the image styles
8. Download the same image processed again
9. Go to /admin/config/media/image-toolkit and set the image quality to 10
10. Flush the image styles
11. Download the same image processed again
12. Note that the filesize and visible quality changed when the image-toolkit quality was changed (a separate issue), but also that the filesize and visible quality did not change when the WebP config was changed...

Proposed resolution

I don't know the cause so I am not sure.

Remaining tasks

- Find out if anyone can reproduce the issue.
- Figure out the cause.
- Propose Fix.
- Review fix.
- Merge.