This happens when I press the Taunt Raider button, but randomly a regular Raider (which appears periodically) also appears a few days later.
Even if I defeat both the elite invaders of the Taunt Raider challenge and the Raider, my achievement won’t be recorded in the military monument.
If possible, I suggest: the regular Raider should not appear for 360 days after pressing the Taunt Raider button.