Stuck after beating the Guardian of the Seal

[Fafhred]

Ok, I beat that boss, moved past it, and see the chest... on the other side of a chasm.

In a previous dungeon there was such, with a switch elsewhere on the map activating a passage (bridge?) to cross it.

Here: nothing, no way; I tried everything, hitting every wall hoping for a secret passage of that very small map.

I still have the key, could not find a way to use it, so I suspet that it is for the chest itself.

I also tried backtracking to previous dungeon areas, even visit new places in the world... dead end so far.

 

I then did a search and found 2 different walkthroughs, the second on this site (https://www.rpginsanity.com/symphony-of ... led-ruins/); both assume that you can immediately get the Underwater Parts after killing the boss, there is no mention of this blocked path.