You may want to re-read the Lottie documentation. Of course you cannot natively export anything that changes the pixel area like your growing arrow tail as vectors. Such stuff would have to be rendered to embedded PNG data inside your SVG, which likely makes it unusable for web use du to file size. You need to work up an understanding of the limitations of web formats and/ or use other tools, if you really want more complex animated SVGs. AE is simply less than ideal for this type of work due to it being meant for conventional compositing first and foremost.