My bottom inserters place the items to the left of the belt.

DerGraue wrote:The behavior of items placed on the belt was indeed changed in 0.16.46 to behave consistently if inserters are rotated.
viewtopic.php?f=11&t=60653
So in previous versions you actually "had" to mirror a blueprint in some cases for it to work, now you can just rotate and it will always work just fine.