Close this as not-a-bug. Found the problem, had construction robots on the belts. Doh.

Reason replacing the underground belts worked, it picked up all the construction bots on the belt. Solution, put a filter splitter prior to remove undesired inputs.

I understand the "chase" but they are not moving at all and sitting on the underground belt. They do work when I first placed everything down.

Here's additional information that I recall from last night. There was a backup of ore to the input of the chain of steel burners. The ore goes to the last ...

0.17.50

I created a blueprint that drops fast inserters on opposite sides of an express belt to insert into a steel furnace. I have multiple copies of the blueprint down with many of them working. I wonder if it is the inserters competing for a resource on the express underground. Ripping up the ...