NPCs can sometimes become invincible.

NPCs located on the upper levels of certain types of monuments often become invincible, making it impossible to defeat them regardless of how many times they are shot from below.
I have purchased all the "dynamic monuments," but this issue does not occur with those. Furthermore, there are no settings in place—such as those used in PvE—that would prohibit combat.
Do you have any idea what might be causing this?

ocean_islandStronghold