Lodged this via EA help and (whilst resolved) was given links to the forum to discuss with others what may have happened - hence the 'general discussion's category.
I refreshed my arena battles back to 5 this afternoon. After confirming 50 crystals the first time, nothing seemed to happen. So I did it again (still said 50 crystals).
Possibly a network / WiFi error on my end who knows, but it ended up refreshing my arena attempts to 5, that was great, but it took 150 crystals out of my balance to do it.
Fauve from EA help was unable to sort it out on their end, but refunded 50 crystals anyway and mentioned something about a scheduled update allegedly causing other issues well
Small issue I know, but as per their advice figured I'd post and see if others have come across similar weirdness.