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:
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.