Mixing IPv4 and v6 for pinging is creating problems and these problems are also different depending on your version of fping.

I have created 4 different example files with different address types and order and went across them with fping and fping6:

for f in pure-ipv4 pure-ipv6 mixed-ipv4-first mixed-ipv6-first; do
for fp in fping fping6; do
echo "${fp}" "${f}";
"${fp}" -f "${f}";
echo $?;echo "";
done;
done

The most noteworthy discovery is that the fping and fping6 commands in version 2 simply bail out of the file as soon as they encounter an address from a family they don't understand. Consequent hosts are not pinged at all; there are no results. Version 3 binaries on the other hand throw errors on unknown family's addresses but continue to go through the file.