I went into the Mine at -9 floor, and unfortunately, I came across a bunch of assemblies. I took care of it before the ARC came and dispatched 2 grenadiers. While I was fighting them, they attacked me for a while. After much gun/grenade firing, it came down to me vs one grenadier. It used to attack, but one time, it stopped attacking. It simply walked back and forth for several turns until it finally starts shooting again.
This problem persists for a long time (even before the current update) because I kept forgetting to post it here. So maybe it's not really a bug? I'm not sure.