1.0.0.1 and 2606:4700:4700::1001 return the same PTR info as 1.1.1.1 and 2606:4700:4700::1111 do.

  $ host 1.0.0.1
  1.0.0.1.in-addr.arpa domain name pointer 1dot1dot1dot1.cloudflare-dns.com.

  $ host 2606:4700:4700::1001
  1.0.0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.7.4.0.0.7.4.6.0.6.2.ip6.arpa domain name pointer 1dot1dot1dot1.cloudflare-dns.com.

I would've expected these to return 1dot0dot0dot1.cloudflare-dns.com.