This has worked as expected for hundreds of flowitems, but then there is an issue with the red-selected flowitem. It reaches the end of the declining section of conveyor and instead of transferring it keeps going through the floor. All data points to it still being "on" the declining conveyor despite being going beyond its horizontal length.
Sorry, I cannot post the model and doubt I can recreate the issue in a concept model but here is a video of it.