In case someone still need it:

Add one more if branch at WebpContainerReader:

if (isFourCc(fcc, 'I', 'C', 'C', 'P'))
                return readIccp();
Also implement this func:
private WebpChunk readIccp() throws IOException {
        int chunkSize = readUInt32();
        WebpChunk chunk = new WebpChunk(WebpChunkType.VP8);
        chunk.isLossless = false;
        readPayload(chunkSize);
        chunk.payload = null;
        debug(String.format("iccp: bytes = %d", chunkSize));
        return chunk;
    }

source : https://github.com/b4rtaz/android-webp-encoder/issues/2#issuecomment-929169092