I'm not sure how to mark it, but this is not in fact a bug. The red circuits on the far lane were all higher quality than what the assemblers would take, but the icons showing the qualities weren't visible, as they were covered by the circuit below them. So it looked to me like the belt is full of ...

I have an issue wherein some of my long-handed inserters will not pick up items (in this case, red circuits being inserted into blue circuit assemblers) when they're on the far lane from the inserter. I checked, and they correctly pick up items from the near lane. Their status is "waiting for source ...