There was a brief window when we first added this experimental feature where SMIL animations were supported in SVGs, but since that type of animation can stutter while javascript executes we followed-up to limit animations that aren’t blocked (CSS opacity and transform animations).
A little more detail here: Branding the Discourse splash screen with custom SVG images - #13 by awesomerobot
So what’s happening here is that the animations are being stripped and the image is being treated as static, and in that case we show the loading dots below it.