a few things

wtfast does not completely eliminate packet loss. you will see a mitigation of packet loss, a significant one, but you will not have zero packet loss. while it is an effective program you should definitely not trust any of its numerical statistics as they're largely marketing points

packet loss to the server has nothing to do with ping

some game servers are proven to be laggier than others regardless of what your 'ping' to them is. aurum vale is regularly a hellhouse of lag when the rest of the game is fine. this isn't just a nin-specific problem - some zone instance servers are just inefficient pieces of trash and minor deviations are more apparent when you're trying to do sub-1s inputs in rapid succession

the problem is NOT specific to ninjas. i can tell you this as someone who raids, someone who's used 3 different vpn tunnelers, someone who mains the job and regularly uses like 3 different zones' dummies to do rotation experiments. it APPEARS specific to ninjas because deviations in server lag are more apparent with mudras, which, again, are half-second input windows that the NA servers do not handle correctly