it's a changeup that seems to have been made to keep people from farming off *t to rank up. The arena event seems to be set in brackets where if the other participants are higher or lower by Xamount you cannot fight them. 
As someone who alt farmed to six paths I understand the why but when arena is already a less popular event this has made it a pain to even bother trying.

Personally I've found ending and restarting every minute 30 to seek a match I somehow get matched... eventually.