While I understand the frustration, I believe this is intended, and does actually fall in line with how the other spells work.
Druid and Knight Coronet’s spells have targeting criteria that can potentially fall on the same troop twice, but since Dark Monolith’s is ordered, this is not possible, so with only one troop left will only hit it once.
The issue here is how the spells are calculated.
Druid’s targets two random enemies, which in practice means the spell resolves twice, picking a random target each time. In theory it’s possible for it to hit the same troop twice even when there are 4 troops on the board. So when there is only 1 troop left, it picks a random troop twice as normal, but since a random number in the range of 1 to 1 is always going to be 1, the same troop is chosen both times.
With Knight Coronet it mentions the first and last troop. And from observing how this (and other spells that target first or last troops) works when there are three or two troops we can see that as the top or bottom troop dies, the game “shifts” it’s perception of which troop is the top or bottom accordingly. So if troop 1 dies, troop 2 becomes the new “first troop”. And if troop 4 dies, troop 3 becomes the new “last troop”. If we have say troops 2 and 3 alive, 2 is the first, and 3 is the last troop. Now say troop 3 dies, so the “last troop” title moves up the line to the next “last troop” which just so happens to be troop 2, which is also the “first troop”. So when Knight Coronet casts, it fires twice, once at the first troop, and once at the last troop, it’s just when there is only one troop left to hit, that troop counts as both the first and the last troop at the same time.
Now with Dark Monolith it’s a bit different. He targets the weakest and second weakest troops. The reason it can’t hit the same troop twice, is because it’s impossible to have a troop be the weakest AND the second weakest at the same time. The weakest troop can’t be the second weakest as well, since being second weakest implies that there is something weaker than you. You could technically argue that the troop should have all the “titles”, including weakest and second weakest, and ones like strongest etc, but then you get into a logic issue where if the troop is the strongest, then it can’t be the weakest, and there for the spell shouldn’t be hitting it.