On the battle map where a unit always starts on the rock, there is a tile which is always out of range.  The image below is Lord Relias trying to cleave a couple of units.  Earlier today, I placed a spearman a couple of squares to the right of Lord Relias (diagonally up from the tile in question) to pierce two tiles, and again that same tile was out of range.

Have also just tried burning hands on the same tile, and again it is out of range.

Edit: performing the cleave on the tile next to it (i.e. between the mites) does actually affect that tile in question (i.e. both mites are cleaved).