Is present at version 1.8.6
dev branch: svn://svn.zabbix.com/branches/dev/ZBX-3030
branches/1.8 revision 21466
"fix versions" says 2.0, but this was only merged to 1.8. that doesn't seem to be correct.
But does this problem exists in 2.0, there is no Unknown events..
Fixed in version 1.8.8 r21466