There is a bug (as you've found) where Havoc won't copy spells where your current target is the Havoc'd target and you're casting the actual spell on something else.
Originally Posted by Eraea
To get around this, I released a macro that got around the problem (most of the time) which is what was shared in that thread.
To explain it, the macro clears and restores your original target if you're casting Shadowburn onto a mouseover target (the conditions for the bug to manifest). If you don't have a mouseover, or it's not an enemy, or it's dead, then it'll instead cast Shadowburn on your current target.
By clearing your target, you are no longer targeting the Havoc'd mob and your spells will cast just fine. It'll restore the target just like you'd expect when using a mouseover.
This macro can sometimes not work if you spam it too much, so don't go crazy with hitting that button.