The most likely cause is a fault rather than a normal setting. On the Canon New F-1, some shutter speeds are mechanically controlled and others are electronically controlled by its hybrid shutter system. If the camera behaves like bulb only in one orientation, that suggests an intermittent electrical connection or internal contact issue that changes when the body is rotated.

A useful check is to try different shutter speeds and see whether the problem affects all of them. If the mechanically governed speeds still work normally but electronically controlled speeds fail in landscape orientation, that strongly points to an electrical/contact problem rather than user setup.

There isn’t an obvious operating adjustment that would intentionally make the shutter do this only when rotated, so the camera likely needs inspection/service.