It theoretically should reset to the rating level you chose when setting up your account. So, if that was in v2, 1200. In v3, it would depend.
As to why it does 400, my guess is it is a bug. I bug reported it a while back but don't know if it really is a bug. If it is, it is very likely very a low priority fix.
Why does my rating for the tactics trainer reset to 400 instead of 1200 when I reset my stats?