Base NPCs melee damage?

I have a nextbot that has an npc_bullseye bound to it.
Regular NPCs can shoot at my bot, but things get weird when my bot gets into melee range.

Combine Soldiers do not attempt to melee my bot, and NPCs like Antlions or Zombies will play their melee animations if my bot comes close, but they won’t deal any damage.

Is there some special way that base NPCs deal melee damage?