I noticed in 3D viewer abnormality in the ground plane.
And indeed, in the board design I found this target. I cannot recall putting it there but it I probably did.
‘it’ does not seem to have any layer control
My questions are:
Kind regards 
Bas
Did you create this target?
The two Kicad supplied targets (Grid & Drill) are not only different but also do not have properties windows.
They are some residual from an old kc capability, they still can be seen in old designs but the UI doesn’t allow you to use it anymore, see:
I may have. The project is what older. It was made originally with Kicad V7 iirc.
Anyways I simply deleted the ‘thing’ and it seems gone now 
KiCAD 5 seemed to treat targets only for the edge cut property. The fill areas were not necessarily kept away from the edge. Items marked on the margin layer in KiCAD 5 also appeared to support fill zones the same way. In KiCAD 8, the edge-cut and margin layers all have “do not fill” tolerances associated with them.
The problem with targets - is that the only way you can edit the layer that they are on is to save the KiCAD project. Close KiCAD, then open up the “.pcb” with a text editor, search for “target” and move it to the desired layer. I prefer one of the fab drawing layers or user drawing layers. Essentially you chose a layer that will not be included in the gerber files you send off to the PCB shop.
A target on the edge-cut or margin will result in:
Note edge-cuts were still for a square box. But you can see the impact of the target on the margin layer.
We really want to set the targets (as default) on any layer except margin and edge-cuts.
For example:
(kicad_pcb (version 20240108) (generator “pcbnew”) (generator_version “8.0”)
(target plus (at 0 0) (size 5) (width 0.05) (layer “Dwgs.User”))
)
As noted in the prior reply - KiCAD 8 doesn’t allow the layer to be selected for the edit. Instead you have to edit the file by hand.
The front side of the board was done with the target on “Dwgs User” the back side show above was with “Margin” the value of the target can be seen when you print a B&W mechanical and select to include the Drawing User layers, but exclude the copper layers.
Using targets also makes it easier to mark the keep out zones and fill zones - which boundaries which can be removed if you had to change the shape or size of the board for some reason.
Targets ONLY on the edge cut layer is a fall back to the days of mylar - missing out that we could use them for other than the pin through the mylar that was done 50 years ago.
