I've always thought that the Frog spell was connected to Frog through Magus. When they were permenantly seperated by time in the end of CT, the curse eventually lifted. I mean, if you're seperated by that much time, in 600A.D. Magus is going to be dead, right? So then the curse should be lifted. But if the gates were open, then the possibility that Magus could come back would still be there. Or perhaps the curse itself flows through the entire timeline and seeps through the gates like water through a crack.
Now, you can either take my flawless explaination, or you can just believe that Magus eventually said to hell w/it for w/e reason and lifted it himself...