From the spec:
For animated raster image formats (such as GIF), the first frame of the animation sequence is used.
The attached test uses a 3 frame animated gif for shape-outside where each of the frames causes the 2 lines of text to wrap differently. You can see the shape animate and it ends up taking the last frame as the shape.
Created attachment 230592 [details]
Test case for bug
Created attachment 230593 [details]
ref file for test
Created attachment 230594 [details]
animated gif for test
Even though the behavior doesn't match with the level 1 spec, we've agreed on keep this "bug" as it is, since it's actually a feature, and will be included in the future versions of the spec.