I have also seen this with the attack-3-adjacent-enemies, where if you have an enemy at positions 8 and 6, and a rock at position 9, you cannot use this ability.
I assume this is true for all abilities, which need to target a tile, and tile is occupied with a rock etc., the target is invalid.
The same thing happens with the restor land-spell, which is more or less useless as you need a 3x3 patch of land, which is empty (no units, shards, huts, treasure chests etc.) and does not contain a single tile of shadowland. As someone else suggested make this is a 1x1 or maybe even a 2x2.
