Pretty simple and self explanatory. Since the old chunks already are already set in stone, there is no treasure to be found. I wonder if opposite can be the case, with old chunk treasure map leading into new chunks.
MCPE-150076
Treasure maps from new chunks can lead to old chunks
Resolved
Won't Fix
2
(Unassigned)
603615, 635348
Community Consensus
Windows
Windows 10 Pro
1.18.0, 1.18.2 Hotfix
Retrieved
Did find 3 old chunk treasure maps, from the old underwater ruins, located down where the marker in first screenshot is pointing at. All 3 maps were exact duplicate of the first one, the one shown in the screenshot - they all were showing outline of the new chunk terrain, despite it's old chunks and as such looks completely different. And also the marked treasure location is exact same for all of them. Exact same map that led me there x3, but for some reason unearthed from old chunks. Needless to say, there is no treasure underneath the ocean.
edit: found 4th exact duplicate of the treasure map in another ruin, which I didn't notice at first because it was generated buried inside of island.