Horizontal hopper chain is moving items too fast for the comparator to send pulsing signal from any of the hoppers input/output flow, the comparator receives a constant redstone signal as long as items are flowing. Note horizontal hoppers only push and don't pull so they should be slower than vertical chains.
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However, we are already tracking this issue, and this report is a Duplicate of MC-75571, which has been marked as Works as Intended - this is an intentional game feature. If you have a different opinion, please visit the Feedback website.
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