I detail and demonstrate the bug in this video:
https://www.youtube.com/watch?v=aKjltyVkd-c [Edit: Accidentally linked Mumbo's video before, not mine]

Using an auto-smelter setup designed by YouTuber Mumbo Jumbo, I've noticed inconsistencies in how many items each furnace receives, often a stack is divided up between 8 furnaces into a 10, several 9s, and one furnace remaining empty for the item-to-smelt slot.

Not shown in the video is the coal inputs for some reason being worse without any changes from Mumbo's design, and usually pooling into the last hopper rather than spreading between them. With the coal it seems adding 1 tick of delay to the repeater may fix the issue, but then that may not be a reliable solution either.