Recreated Furnace Array as in this video: https://www.youtube.com/watch?v=o9gzbMKP4Fc
Items are supposed to flow through the top row of hoppers until there is one in each hopper, upon which the dual edge detector will pulse once to let the items through to a row of hoppers underneath and pulse a second time to let each of those items pass into a furnace. The cycle repeats itself as more items enter the system and they are supposed to be evenly distributed between each furnace in the array. (Explained in the video at 10:10-11:50 and 14:10-15:35)
However, the fourth hopper in this row (Marked in Image 1 by an item frame) never passes any item to the furnace beneath it despite the hoppers before and after it properly receiving items. Nothing is special about this hopper or the one below it, there are no sources of power that wouldn't affect neighboring hoppers in the row as well. This results in an uneven distribution, as each furnace should have 8 items each after a stack of 64 went into the system.
The person who designed this Furnace Array brought up this bug during his tutorial (4:07-4:57) and claimed to solve it by removing and replacing those hoppers. However, this solution hasn't worked for me and commenters have said that when it did work, the problem resurfaced upon relogging. Another commenter said that this bug was fixed in 1.8.6, but it is still here in 1.12