Inserters that start in sync should stay in sync when depositing items on belts, provided there is adequate supply from chests and no bottlenecks on the belts. However, inserters at or near chunk boundaries will slowly fall out of sync with the others at a rate of one tick per cycle. Exactly which inserters are affected seems to depend as well on the stack size of the inserter as well as the orientation of inserter. This video demonstrates the phenomenon with a stack size of 12 items and of 7 items.
https://streamable.com/bto7h
The cause of the desync is a discrepancy in the placement of the first item in the stack. Most inserters will deposit the item very close to the the location of the gripper on the inserter arm. However, the desynced inserters deposit the items slightly downstream (in the direction of belt travel). For express belts, this means the inserters only need to wait 2 ticks before depositing the second item. Subsequent items are then dropped once every 3 ticks. Other inserters need to wait 3 ticks between the first and second items. This causes a desynchronization that grows by 1 tick per cycle. The problem goes away if the stack size is set to 1 or if the inserter is moved away from a chunk boundary.
