From farer distance, some kind of dark pond or even a hole appears in preview, when block is placeable (green cursor).
This disappears if the camera is close enough, and there is a some kind of clipping distance where this is happening, see a sequence of images:
Strange thing, this appears only if the cursor is green, but not when it is red.
If the block cannot be placed, then the ground simply clips the bottom of the block, but while it is acceptable, this is also not a 100% perfect display
As the issue does not appear after having placed the block (terrain gets formed appropriately),
this issue is rather for the todo list of things to be done when everything else is fixed
