How can this happen? I've gone into the sketch of the downstream feature, deleted all external relations, and still it has some unstated relationship to the upstream feature that causes it to be deleted when I delete the upstream feature. There is NO parent-child relationship between the two. This is infuriating!
When you delete something there is a pop up window prompting you or asking you if you want to delete it or not. It also can contain child things in this pop up box that will also be deleted. Before selecting yes to this pop up box, look closely at the check boxes at the bottom of this pop up window. De-select the "delete children features" or something to that effect. Anything downstream of the thing you delete that has a relation to the deleted thing will remain in the design and feature manager, however it'll have some sort of problem that you'll need to correct. This is a big time and frustration saver.