Randolph
Warlord
Since I have noticed this too, I always thought it was because my religion was the nature one (called leaves something).
I've had this happen with mines also, and it does seem to be a bug (although a pretty minor one). It seems to happen when:
1. you move a worker on an non-burning tile and set it to an improvement task
2. The tile catches on fire, at which point the tile is not listed as having a forest (just fire).
3. the worker completes the task while the fire is still raging.
4. the fire goes out and places a burnt forest on top of the improved tile.
5. FOrest grows up with the improvement still in place.