I think this is a bug, when you have the Flamestrike with the Mouse area Skilled, it works in the Assisted Highlight, if you select the one for @Target it will not work. They have to add both for it to work, they are probably not even aware.
Current solution for me until they fix it → skill the one with target area, then use the 1ButtonAssist for only that spell, so you dont have to deal with the annoying area placement.