It’s always happened, that’s how dynamic spawn works, the difference since a long time back is that you can set your viewport to be bigger so you can see them when they appear.
The long-term plan is to switch everything over to a static/dynamic hybrid, where stuff spawns at game start and is persistent, but can change over time. (e.g. animals die off or reproduce over time, or perhaps some of them zombify, but the populations stay roughly constant) Same for fungals, triffids, etc, who would all tend to expand their territories unless held in check somehow.
