The first thing I noticed was that sometimes, searching for kusaka_souji would give me a full 13 (I think it was 13) pages of images, and that sometimes it would give me exactly one image. The one image is post #641290 (sorry, I was trying to figure out how to properly use the curly brackets (also using \ as an escape character doesn't seem to work =/) to no avail). Note that this image does not actually have the tag "kusaka_souji", but rather the tag "kusaka" I have tried editing this image so that it has kusaka_souji, but it does not stick. Removing "kusaka" and then adding "kusaka_souji" also does not work, as it simply shows "kusaka"
Secondly, with the help of flamingspinach, I have found that there is an incongruity between the servers hijiribe and sonohara. Note the difference between the following links:
http://hijiribe.donmai.us/post/index.xml?tags=kusaka_souji
http://sonohara.donmai.us/post/index.xml?tags=kusaka_souji
The former looks OK (to my untrained eyes), but the latter is obviously wrong.
I would submit a bug report to TRAC but I don't know how to word it in a way that someone won't simply close it with "worksforme".
Updated