QS_SrvMaxConnPerIP does not work properly for IPv6 clients. All IPv6 clients are treated as if they had the IPv4 address 255.255.255.255. This can be observed in the output of mod_status. The result is that all IPv6 connections together count against the limit, and thus random IPv6 clients will hit the limit without actually opening too many connections.
Tested with 10.13 from Debian sid and 10.8 from Debian wheezy (source packages compiled and tested on squeeze).
This directive appears to work fine for IPv4 connections. I have not tested other setups, since I noticed this on a production server and couldn't mess with that.