The issue is the same as
ZBX-2394, but applies to the current version.
Suppose we have a host that is pingable by both IPv4 and IPv6 (i.e., by both fping and fping6). Then the debug log will say the following:
7992:20140219:170128.182 Host [www.google.com] cnt=3 rcv=6 min/max/avg=0.000000/0.015390/0.007523
Since we think we received more packets than we sent, the result for "icmppingloss[www.google.com]" item is -100%.