Cyclling through a list will produce a soft target. After acting on that target, you will revert to your hard target. This is a design feature, intended to make it easy to use one-off actions (such as single target party buffs) on secondary targets and then switch immediately back to your main target.
You can convert to a hard target by pressing "Confirm" (default numpad0) before acting on it. You can rebind "Confirm" at
/keybind
System tab