Somehow, in my world, this 1 specific belt doesn't show the item preview after going through belt merger. The item preview works fine before the rotator, after the rotator, and even when I hover the belt merger. Yet, when I place the belt merger, the item preview doesn't work.
This problem somehow got propagated even through a series of stacker and space belt, but somehow got fixed when I stack it after the space belt.
Recreating or copying the rotator & belt from the belt with item preview also doesn't help.