I'd expect the IPv4 address to be considered part of that network (or actually parsed as an IPv4Address and not IPv6Address) even if it's written in IPv6 notation. It's an IPv4 after all.

Python 3.5.1 (default, Jun  7 2016, 09:20:44)
[GCC 4.9.3] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import ipaddress
>>> ipaddress.ip_address('::ffff:127.0.0.1')
IPv6Address('::ffff:7f00:1')
>>> ipaddress.ip_address('::ffff:127.0.0.1') in ipaddress.ip_network('127.0.0.0/8')
False
>>> ipaddress.ip_address('127.0.0.1') in ipaddress.ip_network('127.0.0.0/8')
True