Puppet 7, Facter 4.0.46.
Same problem as
FACT-2886, but this only happens for me on my IPv6-only Vultr VM, so I'm not sure if it's the same root cause as FACT-2886. My other nodes have IPv6 and IPv4, and their networking.interfaces works just fine.
See attached logs.
Note that the fill_io_v6_info! method ("io" is probably a typo for "ip" in that method name, btw) doesn't produce any debug output.
Also note that even though facter networking shows the interfaces fact, there are a bunch of facts missing: ip, ip6, netmask, netmask6, network, network6, both at the networking level and at the networking.interfaces.<interface> level, and primary.
These facts all worked fine with the Facter 3 version included with Puppet 6.19.