I have a Make Bumper two tiles from Fit Rear Bumper and configured to Only Import Locally and it doesn’t. So I’ve added a Mini Supply Stockpile in between both and configured to Only Import Locally and only rear bumpers and it has the same behaviour it doesn’t import.
One thing I’ve noticed on the Make Bumper is when you move the mouse over the inventory it shows the name of the item it’s there. Out of the 12 items, 10 are front bumpers and 2 it doesn’t appear anything.
If you need the saved game let me know.