Now that I know I can fix this by manually editing the opf, I have no problem, until I go back into Sigil and edit an epub for some reason. If I do that, Sigil removes my start page definition. I'm not really sure why but it does. Now I'll need to make sure I remember to do that every single time I find a need to go back in and edit an epub. I think I'm going to put this one in as a ticket and see what happens.