No, it is definitely not a graphical issue. I had this happen to me yesterday when I decided to use staff of renewal on my Fire TB. I spent half the game experimenting with the bug too. Ring of Fire leaves a 10 second AoE effect wherever it is cast. The cool down is usually 10 seconds, which usually makes it impossible to have even attempt to cast a second AoE while there is still one active.
BUT if you have enough cool down reducing flags/items, you can try to lay down a second Ring of Fire while the second one is still off burning somewhere on the map. I haven't done enough experimenting yet to see exactly what happens when you do that, but I know that with a staff of renewal (which gave me an 8 second cool down rather than 10 seconds) I was just BARELY able to cast a second Ring of Fire while the first one was about to burn out. By the time I was done casting it though, they both just fizzle out and no damage is done to anything in the area where I cast the second one.
I'll have to see if I can get my cool downs reduced by like 50% and see what happens then. That would let me cast the second one while the first one still has 5 more seconds to burn. I'll see if they both burn untill the first one ends, or if the second one just doesn't get casted at all.
