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

see also issue20446

Also, it should be marked explicitly, if cross-type lookup is safe. I mean code like that:

    some_net = ipaddress.ip_network(some_input1)
    some_addr = ipaddress.ip_address(some_input2)
    return some_addr in some_net

Is it safe to write such code? Docs say:

    21.28.2.3. Operators
    Address objects support some operators. Unless stated otherwise,
    operators can only be applied between compatible objects
    (i.e. IPv4 with IPv4, IPv6 with IPv6).

1. It is not said if exception will be raised if incompatible objects are used, and not said if result is defined.
2. "Unless stated otherwise" - Seem it is stated nowhere.

https://github.com/python/cpython/commit/5cc274262a99e5633177fc540261abbbecbbc51c

... seems to implement the principle in this bug, but I'm not convinced this is a good idea.