I've had this happen a few times but I put it down to rebellions triggering, however I just had it happen on turn 4 in Gaul, a rebellion would have required around a -80 opinion change in that province but as soon as the Suebi left Gaul appeared with a four-stack army.
Is this a bug or is it meant to happen - because it was basically a rebellion out of nowhere, and it looked like they took the settlement and THEN spawned the army, not that the army moved into an ungarrisoned settlement.
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