In the Advanced section of an Asset Field configuration, there is the "Allow self relations" checkbox. What does this do?
I've noticed that when I have the Propagation Method of the field set to "Save blocks to other sites with the same language" and I have a SuperTable with an Assets field, I need this option to be enabled on the Asset in order for my asset content to propagate if I choose to "Add a site" an Entry (the Section's Propagation Method is set to to "Let each entry choose which sites it should be saved to").