It's actually not unusual at all. You have just discovered how spawning works in atys. In fact, all mob spawns work in this way for all mobs in Atys (almost all? vorax near supernodes seem to spawn differently). in some areas there are several spawns of the same species, which makes it seem like they spawn randomly. Once the last mob in any spawn dies and disappears or is looted, the whole group will respawn. If you are a forager, this is handy information to use when you are digging in a dangerous area with lots of aggressive creatures roaming about.
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