Flushing the playback pipeline when very first buffers are being pushed downstream causes STREAM_COLLECTION event to be dropped before reaching decodebin3. As a result playback never starts after flush (buffer push return not-linked). This happens on initial seek sometimes, when it is triggered at the same time that first buffers push. STREAM_COLLECTION is dropped by GstBaseParse element specificaly that is caching events and sending them with first frame. Unfortunately flushing will drop all pending events if first frame is not ready yet. See: https://github.com/WebPlatformForEmbedded/WPEWebKit/pull/1197