I have never bothered digging into it, just noticed a pretty irritating rise in bot gates after enabling IPv6 through the router (though it could be entirely coincidental). I of course still have an IPv4 address.
Walmart uses a litany of external services, presumably including real-time threat/bot analytics. For instance AdobeDTM, which does indeed serve via ipv6. It seems possible that IPv6 could be playing a part regardless of the status of the base site. These bot gates aren't at HTTP responses, but are in client interrogations and javascript triggers while interacting with the page.
If your ISP uses CGNAT for IPv4, then Walmart could fix the captcha problem by supporting IPv6, where your address is distinct from the bots.