Was this just after a 'cold winter' event, or maybe the deepening?
The deepening ritual cast by the illians usually comes much later than 99 turns so it's likely that the issue is due to the former rather than the latter. If it's due to the event then the terrain should revert back to normal in about 10-15 turns I think?
If you hover over the terrain type then it should state that the terrain reverts to whatever it was before in x amount of turns. This may require going forward a turn to see this though, not sure if this comes into effect the same turn that the event is applied.
The deepening is a permanent climate change for the tile however (I think..) so that tile won't revert back if its due to that. You should have a notification in the event log if the deepening is completed though.