Randomly reading through pixiv dictionary site of Hibiki, and just found out the reason the 25th episode's swimsuit scene didn't have Akira joining them, is because Takatera knew there have been sexual comments about her character and to some extend the actress Nana Akiyama, who's also a gravure idol at the time.
Showing her in swimsuit would just amplify it.

https://x.com/taka_69s/status/508058845650751488