With INPUT set to DROP in both iptables and ip6tables, IPv4 curl works but IPv6 curl doesn’t

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Hi all,

When I access my site using curl -4 www.xxx, it works even though the INPUT chain in iptables is set to DROP.
However, when I set the INPUT chain in ip6tables to DROP, curl -6 www.xxx no longer works.
Both iptables and ip6tables have identical configurations. Could someone help me understand why IPv4 traffic is allowed while IPv6 traffic is blocked under these conditions?

(However, when I set the INPUT chain in ip6tables to ACCEPT, curl -6 works, but this is not the desired behavior.)

Thank you in advance for your help!