Get quest from desalination manager.
Next, get quest from Steel Vigilante.
Maintain all the four Pipe.
After that, the Desalination manager will give you coin and reset the quest (not bugged).
Here is where the bug happens. After talking to Desalination manager, talk to Steel Vigilante.
Zero kill counts.
Still zero kill counts and yet the NPC give coins and reset the quest.
I've done the opposite process, where I talk to Steel Vigilante first and it's not bugged.
This bug also happens in other NPCs like Steel Vigilante (rockridge 356, 156), Mr.McPhy (rockridge 342, 61). The said NPCs will give rockridge coin without killing mob and reset the quest.
Desalination Manager -> Other Npcs mentioned(in any order doesn't matter) -> Clean the Pipe -> then go back to Desalination manager -> then the other npcs will give free coin and reset quest.
Edited by L E G O
Found another bugged NPCs