How to fix "blocky pixelated low poly" DLC models - WoefulWolf/NieR2Blender2NieR GitHub Wiki

Select the problematic object in Blender.
Enter Edit Mode, select the problematic vertices, press M and choose By Distance.

Now to restore the UV islands splits, go to the UV Editing workspace.
Run the Seams From Islands operation under the UV section.

This should have these red lines (seams) appear on your object.

Select one of these seam edges and run the Select Similar - Seam operation.

This should select all the seams. Now press the V key and Right-Click (to cancel any transformation).

This process is recommended whenever a person uses the Merge operation that might join UV islands as well.
To fix visible seams that may occur at split seams, look into custom split normals (no wiki page as of now).