Yeah that's the tank's targeting logic. Try KA's manual mode, it disables quite a few sections of the KA ini including DPS, so you can select any target anytime. One exception, even in manual, if the tank sees a Named, it will try to swap and lock-on. /mqp shuts KA off, so it is best way if you're doing something where you don't want to kill the named first for some reason (like End of Empire mission.)
If you want to use /mqp, then you just don't un-pause until you're ready to let KA's auto-target selection take over again. In End of Empire, I /mqp on my SK as soon as the event starts and leave it off for all waves until both Soulfeaster's are dead. Then I hit /mqp again to toggle KA to take over tanking and grab the Named.
Personally, I bound a /mqp hotkey to my 3 key on all my KA toons. So if KA does something I don't like, I can shut it down and turn it back on once I'm ready. I press it a ton on my tank since that's who I'm usually playing, but I find it useful on all my toons.
You can also do "/mqp on" or "/mqp off" if you don't want it to toggle. I use those for stuff like moving my whole group with zero-chance of a toon deciding it needs to stop & cast something.
Some other useful hotkeys in microscope's
newbie post and
hotkey post.
.