I've just come here to post the same issue and verified on a fresh install that this not just an issue with our instance. I have been doing some experimentation and the issue lies in the iframe that the SVG is placed in to. There is a CSS attribute called "color scheme" that has two values: light and dark. Having either of those set forces the background colour of the iframe to be either white, or dark mode. The SVG itself isn't broken, the transparency is just showing the underlying background colour. I removed that attribute in the CSS editor and all is well now: I would have no idea how to override that for now though.
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