Hi Dan,
I tried -zpad from 20 to 100 in increments of 10. The cut corners gradually come back with increasing -zpad. Turned out that I need a minimum of 70 to completely recover the corners.
I also tried -cubic and other interpolations but those also need a zpad of 70.
One thing I failed to mention thinking it to be not important is that the dimensions of the volume are 192x192x8 voxels (as these are images of the spine). I am rotating in the 192x192 plane so I thought that the depth-dimension didn't matter. Could that be the reason why I need so much padding?