I've tried on the edge, closer to the edge, right on the edge of falling off the edge, under the edge and in short, nothing I can see to how I can select these hidden terrain sub-tiles to turn the terrain back on. Can't turn on water either as I have to be able to select the sub-tile to do that

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As you can see it's in a Tile corner so I can't do a global sub-tile select either! For those that don't know, a 'global sub-tile select is you select a sub-tile (it outlines in red) for example a corner sub-tile, you can move to the diagonal corner on that Tile and while holding the shift key, left click on the sub-tile, and all the sub-tiles on the tile become selected.
This is a welcome carryover from the MSRE and is useful for 'Randomizing' the sub-tiles on a Tile. It also can't be used in this case as the blank tile is in a corner. If it wasn't, I could simple select the sub-tiles on each side of the blank and do the global thing. But . . .arghh...
Any guidance would be appreciated.
using v0.69481
regards,
vince