It's been a long time since I played with targets, but as I recall the implementation of Lua targets is intended to only track the current
target of an Actor. If the Actor changes targets then the targetChanged event fires for that Actor and all references to the prior target become nil and eventually get garbage collected. You are supposed to implement an event handler for the targetChanged event and update your control to the new target. So, the target control is meant to track the current target of a player or mob, not stay on a specific actor. There may be a number of reasons why the devs implemented it this way but I suspect it was simply the easiest way to hook into the underlying targeting mechanism - basically the same thing raid target assists do (they probably piggy-back off that same code).
So, for your plugin to work, you would need distinct actors staying targeted on the mobs you are interested in tracking and not changing targets. I suspect that would not be desirable for your plugin.
It seems you are trying to create trackers that work much like the windows the game creates when you press "H" - a small window that stays targeted on the item or mob you have target at that moment. I wish the Lua implementation worked that way, it would have been far more useful