Trackback url in Subject field not passed through spam filter

I'm finding that the URL behind the Subject field of trackbacks is not getting passed through the spam filter. In other words, if the trackback is from spammerurl.com but the subject text or trackback body does not contain that URL, the spam filter does not catch the trackback as spam.

For example, bardak.com.ru, which is flagged in our URL filter on this site, is hitting one of our sites with trackback spam. The URL is behind the Subject, but the subject title simply repeats the node's title. Like this: Something funny happened at the store today. No spamming words or phrases are anywhere in the trackback -- just the URL, which does not seem to get caught by the spam filter.

I'm not sure if this isn't a spam module issue, so I will cross-post this with the spam issues section. If this bug does not pertain to this module, please close this report and accept my apologies.