Because there is a lake in the middle of your woods polygon, the lake rendering is a combination of lake and woods. A better way is to add the lake as an inner polygon to the wooded area. I also notice, not all of the area is wooded, especially just north of the lake. Multipolygons are more difficult so if you need help, you might check out https://wiki.openstreetmap.org/wiki/Relation:multipolygon.

Can you review this edit to improve it?
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