The 3D interpolation is not correctly triggered / using the correct slices as expected.

Example:

• create a new segmentation
• turn on 3D interpolation
• draw in one slice -> nothing happens (as expected)
• draw in a different slice -> would expect interpolation to start, but nothing happens
• draw in a third slice -> interpolation is triggered, but only uses the second and third slice
• press "Reinit interpolation" -> the interpolation reinits and now included all three slices

It is a bit finicky in general, needing "reinit interpolation" from time to time.