I have a save from 16.12 where I have two stack inserters saturating a beaconed copper cable assembler. When loaded into 16.16 the throughput of this drops. After investigating I found that it was caused by one of the inserters pulling from the inside of a curved belt and failing to pickup items. When the belt is turned the other way the inserter works again.
Thanks for the report. Inserters have always had trouble picking up from belt curves because the amount of items on the curve are small and move beyond the curve quickly.
If you don't want to deal with that you need to not set them up to grab from corners.
If you want to get ahold of me I'm almost always on Discord.