I was trying to find out if Godslayer final form has legs or not (which he hasn't) and by doing some movement to the edge of the arena I was able to get out and fly away from the boss fight.

Then I went down to the island and by manually saving and reloading I was able to maintain the moon bow for the rest of the game.

When I returned to the boss there was no music or cutscene, I had to reach the top of the tower by myself and the fight countinued from where I left it.