If everything has fuel, the mod calculates the shortest duration until the inserter, target, or pickup could run out of fuel items at maximum consumption rate. So if you have chains that are longer than the common ore-to-steel setup, the burner of the target might run dry after another inserter stole it's fuel before the inserter fueling it is checked again.
There also is a maximum delay of the maximum of the 3 "Delay when" settings + the minimum of the 3 "Delay when" settings + "Delay variation" setting.
That leads to a maximum delay of 43 seconds at default settings.
That's perfectly fine for steel smelting with chains of two smelters (my main use case).
You can try to set the 4 map settings with "[s]" in their name to 1 and change "enable extra leeching for" from "all inserters" to "item-fueled" (saves UPS if you only use burners for fuel leeching).
If that works for your use case, I consider an extra setting to limit the delay for when fuel is used and everything has fuel.
I consider making the leeching stock up to vanilla counts.
Currently, leeching happens when the fuel inventory is empty, and it's always a full inserter hand if available (which is maximally 3 items for vanilla burner inserters with full vanilla research).
