Multiple times now I've won an arena round and the game doesn't count it as complete.
How are these bugs not being addressed? Did the entire X-legend team off themselves so there's no upkeep being done? Seriously, fix the bugs before releasing new content.

I find the arena daily tends to break if you win your first match, other than that I've not had an issue with it. Things I can suggest are not opening your mail with your reward inside, and making sure you click the event to ensure it's at least tried to register it.
It does need looking into, though.