When inserting directly into a splitter from the side it get inserted after the splitter if the splitter is facing west / north, resulting in the splitter not splitting the items between the two lanes.
When the splitter is facing east or south the items gets split between the lanes.
In 0.12.31 most of this has been fixed, but the issue remains present for inserters that are behind the splitters (in all orientations). I've filed a new report.
Also, a note to folks who are updating: If you saved a game in < 0.12.31 with inserters stuck dropping onto splitters, after updating I found that you may have to manually walk over the splitter and pick up the items on it to get things moving again. After you do that, it'll continue working smoothly with the update.
Took a break from 0.12.29 to 0.17.79, and then to ... oh god now it's 1.something. I never know what's happening.