I've certainly seen the spare male lechwe in the paddock you mention - think that was two visits ago so 2008. When I went last year they were in Passage Through Asia.
I have a feeling the male Lechwe may have felt lonely and leapt the fence from that smaller paddock into 'Passage through Asia'. I think there are two of them? They/it has been in there sometime now, they obviously just leave them to it.