Overview
Smelter rows in the picture have ratios to fill a bit over full blue belt, yet except for last one, they don't.
Smelter rows 1 and 2 are most typical beacon sandwiches, no.1 uses stack inserters as output, no.2 fast inserters. They sometimes still leave gaps (didn't catch any in picture for no.1, but they can be seen in save file).
Since ratios were correct, I've tested inserter's throughput for a single lane to see if they are the problem, but in this example even just fast ones were enough to fill up blue belt lane. Since chests are refilled to 100, inserters always grab full stack, whereas in case of smelters it's more erratic, but overall throughput should smooth out over time anyway:
Inserters throughput
Smelter row 3 uses pre-compression-changes method of merging two inputs by splitter at the end, 14/2 gives odd number of outputs per lane before merger, so one belt gets 3-4, another 4-3 and then they are supposed to merge to balanced 7-7. It can be seen with debug gap visualisation that right before the splitter inner lanes are fully compressed at all times, yet splitter somehow lets gaps through. This does not happen in the same setup in row 4 when there is bigger distance before the splitter. It seems as if there was not enough buffered items before the splitter in row 3, but debug shows otherwise.
Issue with splitters can be further examined in picture below where loading into splitter is used, some gaps seem to be ignored when going through it, despite inserter being ready to unload:
Splitter trick
The biggest problem is, these gaps still happen even in most common compact beacon sandwiches, regardless whether using 2 output belts and splitter or just a single output lane (slower setups for red and yellow belt seemed to work properly though).
