This happens when a citizen sentinel of type player npc, damages a mob, "drop_money_on_ground" it's set to "false", one real player does the killing blow and you're using TheNewEconomy plugin. I can't repoduce it with EssentialsX eco for example.
So not sure where to report this.
https://pastebin.com/8WgHDYMe

• Min plugins to reproduce:
  -Citizens-2.0.26-b1815
  -Sentinel-2.0.1-SNAPSHOT-b336
  -TNE-1.1.9.2
  -Vault 1.7.3
  -MobHunting-6.9.9
  All in default config but the "drop_money_on_ground" set to false.
• Steps:
  -create a sentinel npc ex: /npc create dude --trait sentinel
  -add monsters target: /sentinel addtarget monsters
  -spawn a monster mob
  -wait for the npc to damage it
  -kill the mob as player

Thanks.