Well, it seems like it's mostly unrelated to belts, and is a mathematical/computation problem of the assembling machine, instead.
It seems complicated, to me, because I can't perfectly narrow it down without being able to see data in plain-English that I want to see (like the formulas and variables at a particular tick for a particular machine). However, I feel confident that it is a bug, and an unusual one which I can't interpret or explain well so it seems like it's "not a bug" on the surface,
much like the battery charging bug I found, which I also extrapolated to (seemingly) some type of math issue.
The issue (seemingly) only occurs on an inserter if it is pulling retrieving from a belt for an ingredient, but that's where belt contribution seems to end.
The issue seems to stem from math of the assembling machine, which it then uses to instruct SOME inserters on what to do. It seems that the assembling machine stores a numerical value of its previous production run up through when its output was full and if the output produced was either an even number (it produced, say, 8 items), or the resulting "full output" was even (resulting in, say, 18 total, ready to be removed).
In a reproducible scenario, manually inserting ONE additional iron plate after it finished its production run and is waiting for output room, does not disrupt the "working" flicker UNTIL the next run. The flicker is nonexistent after the next production run, and an odd number in the output slot is visible. If left to run a third time, the flicker returns.
- Changing the speed of the assembling machine seems to affect it; very slow "normal" speeds don't seem to matter.
- Not all inserters of the same ingredient are necessarily affected--not in the scenario I saved, but in another I was able to have multiple input inserters for wire from belts, but only 1 of those 3 would flicker, same for the iron inserters when there were multiple. (before I narrowed it down to assembling machine even/odd output slot and/or production)
- When some inserters were affected, one time only the inserters at the end of a 2-belt-length line were affected; another time, 1 of 3 on each side would flicker, both being the "middle" of the 3 inserters, but all were fed from independent infinity chest/belts at a length of 1 belt.
- Recalculation seems forced upon the belt-based input inserters on the tick when an output inserter pulls from the assembling machine, and when the assembling machine continues to produce the ~4 surplus ingredients after the input inserters have stopped.
- There must be a surplus of wire before the flicker will begin, even if the output is producing steady even numbers.
- The "math" of the inserters and speed of the machine seems to affect it; a certain surplus threshold or ratio must be met on a particular tick, possibly, because a small iron surplus with even output won't produce the flicker...this can be seen by changing wire input to be belt-based, unless multiple belts are added and a wire surplus exists.
- I didn't try hard, but I don't think the issue is possible with vanilla smelters and their "hard" upper-limit, which may be the same as chemical plants and centrifuges, so the issue may be strictly isolated to any "production" machines which have a "soft" upper-limit of production which can be exceeded, such as assembling machines.
Because it's only on high-volume assembling machines, it has me wonder if that "working" flicker would negatively affect UPS on megabases where machines run quickly with high inserter input values when the output coincidentally ends up "even". I'll try and make a megabase of it to test, but I bet there's a much faster and easier way for y'all to determine if the "working" status chance the assembling machine instructs upon the inserter negatively affects ups.
It seems like there are maybe 6-10+ ticks/instances during which at least one input inserter per assembling machine is "working" when it isn't actually working.
Scenario attached.