Please take a look at this multypoly:
https://www.openstreetmap.org/way/40968499#map=14/47.2958/9.3892&layers=N
There are numerous inner parts that are not cut from the main poly. It should show clearings in the forest.
Like this one: https://www.openstreetmap.org/way/274541206#map=17/47.30263/9.39638&layers=N
Weirdly enough, the clearing is showing up only at z >=18.
I already reported an error https://www.openstreetmap.org/note/840918#map=16/47.3026/9.3906&layers=N