Sometimes ipv4 address instead of ipv6 address
There is nothing wrong with that. Since your website resolves to both IPv4 and IPv6 addresses, the browser (or whatever HTTP client) at the other end may pick an address in random order, or prefer IPv6 over IPv4 (or the other way round). It may very well connect over IPv4 sometimes, and IPv6 at other times. The behavior has to be browser-specific and implementation will vary a lot from one software to another. Maybe this post can help too.