i’m not ericsson engineer, but what about incoming HO from a neighbor cell ? If a MS is configured in HR in a neighbor cell, then it might remain HR after HO.
Or maybe mobile-to-mobile calls : when one MS is allocated in HR, there might be a feature that force the other MS to switch from FR to HR (called Tandem Free Operation in my system)
Those are just some leads as I don’t know about ericsson, sorry..