The Tanada House of Fire Quest involves recovering ancient scrolls that belonged to the houses of Air, Earth, and Water. Tanada Sajo explains that, "because Aerbax succeeded in corrupting the Masters of Air, Earth, and Water, he was able to steal the scroll of each House." However, the lore of the Tanada House of Earth Quest implies that the House of Earth, and specifically its leader Master Seijuro, are unaffected by the recent corruption of the House of Air.
It is unknown at this point which is accurate. Certainly, at the time of the Tanada House of Earth Quest, that house was uncorrupted, as the entire plot of the quest revolved around this fact. It is possible that later the House of Earth was corrupted, and the quest was simply never updated to reflect this (as is often the case with quests as they relate to the overall story). It is also possible that the developer creating the House of Fire quest simply forgot the details of the House of Earth quest, and the inclusion of the House of Earth as a corrupted house was accidental.