When I put 2 splitters there that the fast inserters are going to grab from, only the top 2 were working. I checked the F5 debug hitboxes and they should be able to load the train. Where it gets more odd is if you put a splitter in the dead center. Then both the middle inserters will load the train. But if you put a splitter on the bottom, then the bottom two fast inserters won't load the train.
I'm replacing this with a little buffer system of course but since I stumbled across this seeming bug I wanted to report it.
