Vertical focal plane shutter and camera orientation

Here's an odd question, but I'd be grateful for your views.

Would you consider it normal (or reasonable) for the vertically-running focal plane shutter in a brand new DSLR or mirrorless camera to malfunction with the camera mounted upside-down?  Though I can see it's not a situation they're typically designed for and the effect of gravity is reversed, which could potentially have a result like this, I wouldn't actually expect the shutter to be affected too much.  I had the camera upside down to get evidence of a lens decentering issue and took two exposures.  On the second one, the shutter tapered (capped) - i.e. a narrow strip across the top of the final image was completely unexposed.  The shutter speed was 1/320 - way off the camera's maximum of 1/4000 where I'd perhaps expect such an effect to show up more readily.