As some of you may know there is a bug where material on a conveyor that is offscreen may not transfer heat with the cell that it is in. More specifically, when the conveyor is offscreen to the northeast or southwest quadrants the bug occurs. When offscreen to the northwest or southeast quadrants heat transfer appears to be correct.
The bug seems to be that material on the rail acts as if it's in the cell that originated the length of rail. That is to say the cell with the output of the loader, shutoff, or bridge from whence the material came.
As a workaround, ensure that the output of your loader/shutoff/bridge is in a cell with which you want to exchange heat. For a typical example of running hot igneous through diamond blocks, you must use a bridge that has it's output in the diamond. A loader in vacuum transfers no heat because the game thinks the material is in vacuum regardless of where the rails actually are.
Note that this doesn't avoid the bug, it just minimizes the problem. All the material on the rail transfers heat with the first and only first cell. You must rely on cell to cell to transfer heat further. You'll notice there is a bump above that first cell. This is because when material comes out of the bridge into a solid cell, it jumps above the cell and transfers heat as debris ON a cell, which is slower than debris IN a cell. That bump enables faster heat transfer and again all the transfer is concentrated there.
For your typical loop with an automated shutoff, ensure both the input bridge and loop bridge have their outputs in a heat exchanger cell. Something like this, stolen from Lifegrow :
The more "creative" of you may already be thinking of dirty tricks to exploit this behavior. Anything that melts on a bugged conveyor melts as if it's in that magic first cell. Each 20 kg conveyor packet behaves as a separate debris pile for heat transfer, so 100 segments of rail = 100 piles of debris. The heat transfer of 100 piles of 20 kg is much, much, MUCH faster than 1 pile of 2000 kg. Do your worst and maybe Klei will pull their finger out long enough to fix this very old bug.
With thanks to Lifegrow for being my muse while I investigated the issue. And as always please share any further discoveries.