First, it looks for all the world, (unless you've made all the colors black and turned off printable area and paper background, etc.), like you're trying to put your title block into model space. Is this the case? If so, your "viewport" isn't really a viewport, as I understand it. Your viewport will show you modelspace in a paperspace tab.
...But rereading your post, I don't think that's what you're doing. I do know a quirk that may help: If you leave the layer with your viewport on, (perhaps on a non-plottable layer), you can have any shape viewport you want. But if you freeze the viewport layer it forgets the defined boundaries and reverts to (I think) a bounding box around the extents of the viewport. Could it be something related to this?
