When changing "events logged", in Administration … Logging, syslogd and-or klogd rewrites the Message Log, instead of appending, thus destroying the records in the Message Log.
Furthermore, the Firewall is restarted, thus destroying the counters accumulated since the intentional start of the Firewall. This is not a reasonable side-effect of making a minor change to the events to be logged.
Syslogd should preserve records, including those accumulated by iptables, not destroy them.
The description is plain enough and the bug can easily be reproduced. I've verified it on Toastman Mini for WRT54GL and Toastman 7494 NOCAT on RT N16, both K26.