I submitted an in-game bug report that was moved to the accepted bugs forum. I figured out how to solve the bug, but don't have proper access to update the post or cancel it.

It was for the post titled: Unable to use bootshine on Trial Stone during quest "Harder than Rock"

I figured out that you must quickly select the trial rock and then immediately use bootshine. If you select the rock twice or your character begins trying to attack on its own then the bootshine will always show invalid target even though all the other abilities will still work.

So it is still a glitch, but one that can be circumvented if you figure out the trick. Even having support know this when someone posts the issue will help. I'll keep my eye out for any other posts and make sure to help answer if I have the ability to.