With SpaceEx and latest update for BRL, there appears to be a hidden diagonal inserter that pulls or pushes items in/out of the loader/unloader. If the unloader is placed with the rail heading north, the problem corner is at top left (north west corner). For the loader, it will pull things of a belt running thru that tile, even if the belt is not delivering items into that particular loader. I've tried placing loaders and unloaders manually and the inserter arrows appear as i place belts near the entities. Placing the BRL manually, the hidden inserter is positioned correctly. Copy/pasting existing loaders/unloaders, it seems to occassionally be copied pointing at the wrong tile or somehow gets shifted when placing the copied objects.

I'll try to reproduce it in a clean save file.