I've seen so many people talking about this bug, but I've never experienced it myself. Now granted, my testing is minimal since I've been playing mostly Resto, but I've run at least one instance at Balance and am often Balance when out soloing. I've always been able to cast both Moonfire and Sunfire with no issues, using the same macro I've had sitting in bar for a long time:
#showtooltip
/use [@target, harm, nodead][@targettarget, harm, nodead][] Moonfire
Next time I have some time, I'll try playing with it and seeing if I can reproduce the bug, but for now I'll just give the one data point.