This is because https://www.openstreetmap.org/way/121949883 way had `natural=wood`, but it's actually a member line for the https://www.openstreetmap.org/relation/14346673 multipolygon, which is supposed to be the actual forest. So there were indeed 2 forests in that sense.

In iD, you can spot these sometimes because the filled area appears double as bright - because it has two area fills overlapping. https://imgur.com/a/W7wqRX8 This is usually due to multi-polygons and their ways repeating tags or having incorrect inner/outer members.