Did a little testing (not too much but i think enough)
This only seems to occur when you are matching 'nickname and message' and the highlight words include parts of both, and as the OP stated it only occurs when the user is a regular.
Did some testing using the colors in address book, this had no impack on the behavior, that is... highlighting was applied in the same manner regardless of address book color settings.
In these circumstances the highlighting (at least that specific highlight) does not seem to be applied at all.
Tested on mIRC 6.17 also.
