Keyboard/Mouse user randomly losing selected target

So, I'm a keyboard/mouse user, as stated, and also a healer. Whatever you would call this issue is absolutely KILLING me. Basically, whenever I have a target selected for healing, sometimes I just randomly lose target, and it's pretty game-breaking, because with all the things going on, I fail to notice sometimes, and heal myself once or twice instead of the tank.

I'm not accidentally pressing any other buttons, or anything. I've experienced it enough times to know that my target just randomly drops on its own.

I don't have a controller plugged in at all, if that might have anything to do with these kinds of issues.

Getting the same thing....and also a healer with desktop, keyboard and mouse.