certain components used in the editor are designed to ONLY work correctly to produce seamless results on square images. thus, non-square images tend to produce results like you show in your post. thus, this is a program design limitation. as CF&M mentioned, color gradients are one of those components. and sometimes, the only way to produce a certain result in a given filter is to use those components which do this.
one of the most requested wish-list items is, make it possible to use those components with non-square images without producing the effect you mentioned. but, with FF, inc. being so keen on seamless tiling results, we're not sure we'll ever get this. but, at least now you know what's occurring with the filter.
If wishes were horses... there'd be a whole lot of horse crap to clean up!
Craig