The shipcannon is firing at non existing enemy’s.
For instance I have tree enemy’s left, first has 4 life left, second has 5 life left and third has 3 life left.
I use cannon and it fires at the third then the second and than again on the already third enemy… how stupid is that???
Even more stupid was the following: two enemy’s left, one had 5 life left and the other 4, used cannon and it fired tree times at the same enemy…
On pc via Steam.
I complained about this a few years ago and was told it’s working as intended.
Troops that have multiple effects lock in their targets before going through their moves. I think this was introduced as a Khorvash fix, to prevent the mana drain effect to hit troops further down in case one of the front troops dies to the damage effect. For all we know it’s intentional.
If it’s intentional it’s pretty stupid. 
Ever heard of a gunman who shoots on a target that isn’t there anymore?
Even when there were fears fight on sea with cannons that needed loaded with gunpowder and so on, they weren’t shooting at sunken ships… 
I like to picture it as the shotgun approach. Close range at a single target it will do huge damage, firing at a group will just spread some hurt around in an unreliable way.
Hmmm, I don’t know but isn’t there a huge difference between a shotgun and a ships cannon?
And even then, who is gonna shoot a shotgun on a non existing target?