The land could be Fertile_Land. Both Fertile_Land and Plains are named Plains in the XML. So its entirely possible that you see a creature lair with tile props that doesn't make it look like normal and when u mouse over it says plains-grassland or plains-fallen and once the lair is removed theland that was there is fertile_land.

If you want to check this copy the terraintypes xml to your mod dir and change the displayname for Fertile_Land to  "Fertile" (or something other that plains) Now in your tooltip you will see which land is fertile as opposed to normal "plains" land.