After major updates in Honkai Star Rail, compatibility issues really do pop up frequently, especially if you've already got a fresh version of iOS on your device. iPhone 15 Pro on iOS 18 is a relatively new combination, and sometimes optimization lags behind. On top of that, leftover old config files can pile on, the ones that don't get deleted during a normal reinstall.
Since you've already cleared the cache and reinstalled - try going more radical. Delete the game, then go to Settings → General → iPhone Storage, find Honkai Star Rail in the list, and completely delete the remaining data. After that, restart your phone and download it fresh. One more thing - make sure your device has enough free space (at least a couple of gigs), because lack of storage can also cause crashes during fights.
If that doesn't help, then yeah, the problem's most likely on the developers' end. In that case, it makes sense to reach out to the game's support with a description of the error (usually you can submit crash logs straight from there) - they'll figure out what's going on faster than anyone else. Sometimes they put out a hotfix specifically for cases like this.