ohh no!! How frustrating!
I played through the game tons and that never happened before, was the boss alive or dead at the time of you wanting to pick up your lost treasure?
At 25%, 50%, 75% and 100% in your notoriety gauge there will be hunting ships out for you on the map... if they’re not dealt with by the time you reach the boss it can make for a pretty difficult battle.
If it’s any conciliation if you had of picked up your treasure AND already killed ol Solomon you would have completed it and won.
Thankyou for your time and feedback