I've noticed that the long arm inserter is not able to drop items on a splitter, but the stack inserter can. I'm not sure if this is intended, or a bug.
The long arm inserter can, however, collect from the splitter.
[0.16.51] Long arm inserter unable to drop items onto splitR
Re: [0.16.51] Long arm inserter unable to drop items onto splitR
The drop area is occupied, NaB.
Re: [0.16.51] Long arm inserter unable to drop items onto splitR
Rephrasing what Loewchen said, the long handled inserters are attempting to drop the item after the splitter logic (far side of the belt, or in this case the far side of the splitter), the stack inserters are dropping before the splitter logic.