seems to only happen if las/laz tile has points outside its own tile bounds.
for example laser scanned house:

• scan 1 will have (stray) points in scan 2 area also (through door/window or hallway etc)

so to avoid that:

• Load all points to CloudCompare (or other tools), merge as single cloud
• Then split to real 3d grid (for example Cloud compare las/laz importer has Tiling option)
• Each of those tile will only contain points within the tile itself (not overlapped with other tiles)