I have a revelation that may affect some people's opinions on ultimate arena.
When I claimed the rank 1 rewards, it said I got 10M coins, 5M credits, and 14k rep.
However, when I refreshed my browser, I actually got 15M coins, 7M credits, and over 20k rep.

If this is not a glitch, it means both regular and ultimate arena rewards get awarded. If this is intend how it is intended, then apart from stakes (which can be resolved through dialogue between the guilds), this ultimate arena can only add to everyone's rewards, not reduce.