irule clientssl_clientcert log

Per iRules 101 #9 Debugging:

"While the facility and level parameters are optional, it is good to know that there is a significant behavioral difference when the optional <facility>.<level> is specified. When iRule logs messages without the facility and/or level, they are rate-limited as a class and subsequently logged messages within the rate-limit period may be suppressed even though they are textually different. However, when the <facility> and/or <level> are specified, the log messages are not rate-limited (though syslog-ng will still perform suppression of repeated duplicates)."

Try adding facility and level to all your log commands to see if that makes a difference. For example:

log local0.warning "Client Certificate received: $subject_dn, [IP::client_addr]"
log local0.warning "BLAAT"

You can also use just the facility - local0. - part without a log level and leave it to its default, which is Info.