The dynamic field is used to either process the image through Imgix or not.
In both instances, the original image shows for SVG. It gets processed as it should for PNG images.
By default, Imgix handles SVG images by pass-through, which means they aren’t transformed.
For most scenarios, maintaining the lightweight, lossless scalability of vector graphics is desirable.
Imgix customers must specifically request the ability to transform SVGs. Maybe Bubble hasn’t made that request.
Be forewarned, however, that the SVG will be converted to a different format, and the file size will very likely increase (which of course runs contrary to the whole purpose of image optimization).
EDIT
Just wanted to add that it’s very easy to convert a SVG to another format (PNG is often a good choice) on your end using a graphics editing app (such as Photoshop or Gimp or whatever) and then use that in Bubble. If you really need to do transformations on user-uploaded SVGs, though, that would require Bubble to ask Imgix to enable SVG input. But again, the output files may wind up being larger than the input SVG files.