Is this a bug or expected behaviour?
It happens from time to time and I don't know how to fix it.
Any ideas?

Take a look at the object's parameters. Most likely you have a scale at the object level. Possibly a non-proportional scale. Houdini's SOP handles can get distorted in SOPs if that is the case.

To fix this you can append a Transform SOP and apply the scales at the geometry level and set them back to unity on the object.

There is a shelf tool called “Freeze Transforms” built just for this purpose that will reset the Translates, Rotates and Scales to their defaults and place a Transform SOP and move the Transforms in to the geometry.

I think that was the case… Freeze fixed the problem…
Thanks a lot.