An example: There is a closed ring with natural=coastline and place=island. In order to draw a forest as a partial area, you have to divide the ring. Shorter lines and a new multipolygon with natural=coastline and place=island are now created. The coastline tag must stay on the lines and only place=island and type=multipolygon belong on the multipolygon.

natural=coastline -> line
the description of the island -> multipolygon

You did everything right except for the tags, unfortunately they are all attached to the multipolygon. It happened several times over the last few days. I've already corrected that.