If one wants to close a 2d segmentation, it dissapears because the 1 slice is eroded. This is especially confusing, since erosion and diletation themselves seem to work for 2d slices.
At the very least there should be a disclaimer in the segmentation utilities checklist that a 3d segmentation is necessary for this test.
Closing should not make 2D structures disappear. As Sebastian noted the opening filter does so as intended. On 3D images the erosion filter has the same effect on 2 structures. On 2D images the 2D filter version is used.
What Sebastian described still happened to me during the release check.
As a quick fix the user could be notified that the operation only works on 3D segmentations