What steps will reproduce the problem?
1. click out side of squared, so that no one is selected
2. click on person A's square in squared, to set them as your defensive target
3. move off squared, person A is still your defensive target since you are not hovering over anyone, and they were set earlier.
4. move back onto squared, and the defensive target follows the hover, but upon leaving it once again returns to Person A as expected.
5. Move back onto squared, and select person B with a left click.
6. move off squared, person A is your defensive target rather than B

What is the expected output? What do you see instead?
Left clicking should change your defensive target, even if you already had one set.  It seems to function correctly before you have one set, but not after you set it for the first time.

What version of the product are you using?
1.7 and I have both this and hover cast installed.  

Do you have an error log of what happened?

Please provide any additional information below.
I played about 3 scenarios yesterday with the 1.7 version, and knowing that the first person and the above functionality was consistent.  I did not see any mention in the 1.8 patch about this, but I will try installing it to see if it still exists there.