In the area I am mapping, a previous mapper created a pond, and then in the pond s/he created an island. When rendered, the island doesn’t show up - it looks like the pond “hides” the island. When going into “edit” mode, they show OK.
You have to combine the pond and the island in a multipolygone relation, which can be done easily by using the standard editor iD. Just mark the pond by clicking the outline and additionally mark the island by holding the shift key and clicking the outline as well. When both outlines are marked, press the key “c” and the objects will automatically be combined in a multipolygon. That should solve the problem.