I rescued both Caravan people from the Gnollattack. Then i decided to let Chest where it is after having talked to the Caravan Guy Rugan, who gave us the Phrase for entry into the Hideout. Then i went to Waukeens Rest WITHOUT CHEST. I didnt opened it. But i did on another save just for curiosity whats inthere. So far everythings save and ok. No Chest opened and no Flask stolen and opened.
At the Hideout i was rewarded by Zarys for defending her Agents and bringing back the shipment. What,.... I didnt bring chest because i assumed Rugar can take care of bringing chest himself. Well i didnt thought again of this minor bug in text...
BUT...some Hours later i returned to the Area where i met Rugar. The Chest was still standing there closed as should be. Strangely when i went further into the cave BATTLE started and the Beholder was on the top spot above me right side balcony where theres another normal closed wooden chest.
So obviously Rugar had his own Agenda about the Iron Flask and didnt bring back the real chest including Ironflask, but somehow when he was away the Beholder could escape his Iron Flask on his own by vile magic or whatever. OR thats simply a bug.