When using CanvasKit-bundled image decoders Flutter Web decodes animated images starting from the second frame rather than from the first frame. The issue is that CanvasKit.MakeAnimatedImageFromEncoded advances to the first frame automatically, but Flutter doesn't know the duration of the first frame. So we have to call decodeNextFrame at least once to get the duration (repro). This was recently fixed in CanvasKit (google/skia@44c81d1#diff-a40c963d58980093361179fa6b6c9832abf234df8d3ff257bebaa5d7704f37d0), so we can now fix it in Flutter Web too.

flutter/engine#30224 added a test that covers this behavior, so when this is fixed that test needs to be updated.