Yeah, if Reaper and Cubase both wait for the render, but Studio One does not, it is therefore an S1 issue. The funny thing is that Studio One claims to be the Drag&Drop King, in that you can drag and drop anything to anywhere, effects, sends, tracks, files, and more. So maybe they made it too fast to handle BIAB rendering, which only takes a little while, but it wants everything immediately. It's trying to get an incomplete file.