I have an Oslash as a composite made from an O and slashlongcomb. The slashlongcomb is made as a simple horizontal line that is anchored in the middle of the O and then rotated as a component in the composite. But when I then try to scale the slashlongcomb component so it sticks out more, it is no longer anchored in the center, but scales from the origin point. Here are some pictures:
While this is not relevant to the issue described, I would still mention that I don’t think it is a good idea to compose the Oslash like this, at least not if you have your slashlongcomb set to export. It is a combining mark (albeit exceptionally rare) and should thus follow the expected design.
I keep the slashlongcomb non-exporting. I find this a neat way to re-use the slash because when it’s perfectly horizontal I can increase its width as a component by just transforming it horizontaly and it doesn’t distort. Then I just rotate the component. And if the anchor worked as I expect it to, then I would not have to disable the automatic alignment and everything falls into place.