FYI, I have submitted an official bug report through the Feedback Assistant app. I will let you all know if and when I hear back from Apple.

After using the sample app we created (see last comment) we have identified that this is due to an issue with AVAudioUnitEQ specifically. If you remove that effect from the chain in the sample app, the crash never occurs. It would seem that even after we destroy the instance of the AVAudioUnitEQ effect (when it is no longer needed) there is still a process that tries to reference it after the fact. There may be an asynchronous process referencing these audio effects after the fact but when it does try to find it, the effect has already been destroyed. This does not happen for the other effects and seems to be isolated specifically to the AVAudioUnitEQ.

We have created a minimal app that shows this error in a bit simpler detail. This is stripped down to the base AVAudioUnitEffect classes: https://github.com/audiobridge/iOSCrashDemo

Based on the EXCBADACCESS error, my guess is that some piece of the process gets deallocated from memory and when the code goes to alter the settings on the audio effect it no longer exists. I turned on Zombies to try to capture any deallocated objects in Instruments but it did not turn anything up. I also downloaded the iOS 14.1 SDK and ran a comparison on the audio frameworks to iOS 14.2 (AudioToolbox, AVFoundation, etc.) to see if anything significant had changed but it doesn't appear so so it could be a bit lower level than even these libraries.

Update: I have tried enabling Zombie Objects and the Address Sanitizer in XCode to get more information. Even with Zombie Objects enabled, I still received the same EXCBADACCESS error.