Re: Can't find last memory trigger - Seeking advices how achieve it
The memory quest has many triggers on the worlds, as you have stated. All of them are shown on the map, as far as bugs allow. But the quest itself requires you to keep listening to what the memory unlocks in SAM's node, also, some interaction with your sibling, as both of you sort of 'unlocks' something else together by talking about some events past, you see. These things are usually referenced in your quest log ("Talk to your sibling" etc.) and the last part of this quest requires you have access to the Cryo Bay, so it won't trigger after beating Meridian.
In a site, there's this clue, and I think it's very straightforward:
"The westernmost trigger is located in a cave below the tall stone column. The entrance to the cave is just to the south of the map marker, and the trigger is in a cavern with a Fiend so enter with caution."