I am doing an IR2 playthrough with this mod included. I'm stacking iron ingots on the bus, then unstacking as I draw off this line. Periodically I'll find an unstacking box 'jammed' with one or more iron ingots sitting in the input slot. Only stacked iron ingots are on the belt that feeds into the unstacker boxes, so I can't see how this is user error. Any ideas what could be happening? In the meantime I'll hook speakers up to the input belts to alert if any iron ingots are on them,

No real ideas beyond the obvious stuff - there could be a belt or inserter that accidentally got rotated somewhere, or something else that is contaminating your lines. That's what it's always been in the past. It's not possible that the beltbox itself can be passing iron ingots from its output into its own input slot. IR2 makes zero changes to the way DSB works, it only changes the list of items that can be stacked.

If you can't track the contamination source down, decide if you want to post up the saved game and get a second pair of eyes on it (I'm not necessarily volunteering but someone else might).