In the documentation, No information about the possible value formats for CF-Connecting-IP header regarding IPv6 values and the possibility of getting ipv4, ipv6 values together.
Can you verify the possible values of CF-Connecting-IP header and how to get IPv4 & IPv6.
A request cannot come from two addresses, so in the case of this header it will always contain the original client address. The only exception is when you chose Overwrite for Pseudo IPv4 address, in which case that header will contain that pseudo IPv4 address.
Then the header will only contain IPv6 address. Like what Sandro mentioned, one device can’t connect to another device via multiple IP addresses. Either one only.
IPv6 only devices can - as the name says - only send requests with IPv6 addresses. If you do not have mentioned feature enabled, you’ll only get that address then.