I tend to follow you on that the captive network assistant should not show the Success page, but based on that page should close itself as there is open connectivity.
If you search Google for 'Apple IOS page shows Success', it looks like you are not alone, it is not just Aruba, and it seems something of all times (not just recently).
My personal feeling is that you will need to follow all the steps that are happening, and see what is different. What also may help is to test if older iOS versions do or don't experience the issue, and if the issue is existing on both controller and Instant (I believe you have both flavors in your network).
What also may give additional information is to capture traffic and see what is the difference between your old workflow and the new one on client behavior. And could the timing be part of it? What if you wait for 20 seconds before you 'Accept and continue'?
Please work with your partner and/or Aruba TAC if you get stuck during troubleshooting. And if you find the answer, I'm pretty sure other Airheads will appreciate hearing what you found and what is the solution.