I think it may just be a thing that happens if your Antonio is any good at all

I agree. I think it's possibly partly a problem of time and genre, as is more obvious in the scene in which Malvolia is being tormented in a dungeon as 'mad', which these days in a good production is likely to be really awful and obviously cruel and distressing, but which in the original period would have been Hilarious Japes. I'd love to see a version that managed to give Antonio a happy ending, can't he have a poly relationship with Olivia and Sebastian?