When new enemy spawns, they will only be available to move at the next turn, after every other existing characters have moved on their current turn.
This allows you to manipulate the turn ordering of new enemy spawns. I find it hard to explain in words, so I'm just going to use the graphics below.

It lets you choose, if you want these new enemies to move before us (Scenario 1). Or to move after us (Scenario 2), which might allow us to kill the new enemies before they even get to move.
Anyway, I've found this from my recent project of routing a Suikoden Tactics NG+ speedrun. I've been chatting about the game mechanics on the Suikoden speedrun discord, feel free for any Tactics enthusiast to join.
https://discord.gg/pES93ay