If you guys have a save BEFORE finishing talking to the nobles, there is a workaround that works.
Load that save, clear the hideout just west of Marunath, then finish talking to the nobles, then talk to Arzagos. The hideout will respawn with the banner piece inside.
It seems to be a bug where the old hideout without the banner piece is still there instead of the one Arzagos tells you to clear. If you clear it beforehand, it disappears from the map, and the one with the banner piece can spawn properly.
