Problem with Access to the Pinnacle Boss Encounter

i unlocked access to the Pinnacle Boss through the Expedition mechanic and left the instance to improve my gear before attempting the fight. However, when I returned, I found that I could no longer enter the boss encounter.

The problem is that I have no way of obtaining another entry item, because there are currently no Expedition bosses available on my map (such as Olroth and others).

This was my second attempt at this boss. During my first attempt, I was able to leave the instance normally and return much later (after completing my Ascendancy), and I could still enter the boss encounter without any issues.

Therefore, I would like to ask whether it is possible to restore the entry item, or if there is any way to obtain another one.