Trouble creating multipolygon out of existing area

I don't use the iD editor preferring JOSM so can't comment directly about iD But, the difference between the lake and other areas added to the wood multipolygon, is that the lake is itself a multipolygon. The lake is made up of three parts, (two parts described by the multipolygon and one (the Dam) described on the individual way). One way over this problem is to delete the lake multipolygon, make the lake an area described/outlined by a single enclosing line with of course its own tagging, draw/keep the dam as a separate linear feature, (it can be on-top of the lake polygon. Then add the lake (now a single line polygon same as the other areas) to the wood multipolygon. This method works with JOSM, hope it helps.

Using Josm as I am unfamiliar with iD:
If you wish to leave the reservoir multi-polygon as is you can:
Open a relations window, select the wood multi-polygon, select the relations editor in the relations window, right-click on the Camp Red Cedars reservoir multi-polygon in the relations window and select relation, then in the relations editor add the reservoir to the left side panel, add an inner role to it, then ok and save. I didn't save the above to check but it should work ok.
An image may help...