I had this issue too…first with iPhone 15 Pro after iOS 18 upgrade, then again on iPhone 16 Pro. Narrowed it to the exchange server connection I use for work. Work requires a Hub app that installs a profile used to confirm 2 party authentication. Calendar would freeze, regardless of exchange calendar or iPhone calendars. That would put the phone processor into overdrive, heating it up while draining the battery. iPhone restarts stopped the drain, but who likes rebooting that often. This affected my iPad Pro M4, too. So two iPhone versions & an iPad. I have another iPad w/o the exchange connection that has operated w/o issue.
My solution - completely removed the Exchange connections. A few reboots in between, then reinstalled the exchange connection. That solved my problems. My calendar app is working as it should with all the new functionality if iOS 18. I’ve read on other posts whom have done similar actions, but had the issue reoccur later. But so far, I’m okay (knocking on wood as I write this). It’s weird. Most of my work colleagues use iPhone and the same exchange connections. I’ve not heard anyone else complain about this issue. However, I’m normally one of the early adapters of new hardware and software releases, which maybe explains why not many others have complained about this issue. Keeping my ears open…hope this helps you.
