Factorio Forums

Steps to reproduce:
1. Alt-mode off

1. Pick up on cursor Electric miner (item or ghost)
2. Pick up on cursor Burner miner (item or ghost)

What happens:

1. Existing miners (entities or entity ghosts, burner or electric) show no arrow
2. Existing miners (entities or entity ghosts, burner or electric) show output arrow

Expected:
Consistent behavior, which is to say either arrows both in case A and B or no arrows at all.

Interesting catch, but i will go with "Not a bug".

Arrows on miners are turned on when you hold entity that can receive stuff from other entities to help with deciding where to place your building so it will receive stuff. So if you hold belt or assembler, these arrows are also turned on.
Electric miner will only output items so there is no reason to show arrows of miners other than one you hold. Burner miner can receive coal and so this arrows are usefull when building initial burner-miner coal mines using loop of burner miners.

(only exception i found is "radar" that turns this arrows on when radar itself cannot consume any items)

Thank you for taking the time to write up the bug report.

@boskid is correct, the arrows are shown because burner mining drill has an inventory accessible by other machines - its fuel inventory.