I'm afraid there are a few small areas like this around the Kavieng area of New Ireland, & the N coast of New Britain.
Fixing this is difficult as I created these sceneries back in 2013 & don't have the source files for the Land & Waterclasses anymore.
I have created some patches, but they conflict with the originals, & this can sometimes cause CTDs during transit, as the sim gets confused with which file to load.
This can be avoided by putting the patches in a separate layer of their own, or putting them in the "Reefs" or "Rivers" layer, but a more elegant solution might be to separate out both 2013 sceneries from the whole SWPTO scenery grouping & create a grouping just for New Britain & New Ireland.
I'm still experimenting, as I wonder if there are issues (performance/stability) in adding more & more scenery layers.
I have even begun to separate out the airbases from the terrain & put them in another layer, as I now do with my most recent works.
I can then activate/deactivate airbases depending upon where/when I want to fly