Addendum: Loading samples from NSDocumentDirectory instead of the bundle results in the same behavior.

Ok, it turned out that is was not a bug in AVAudioUnitSampler - running the same code on macOS gave some additional hints... unable to obtain configuration from file:///Library/Preferences/com.apple.ViewBridge.plist due to Error Domain=NSCocoaErrorDomain Code=256 "(null)" UserInfo={NSFilePath=/Library/Preferences/com.apple.ViewBridge.plist, NSUnderlyingError=0x600003a9e610 {Error Domain=NSPOSIXErrorDomain Code=24 "Too many open files"}} ...with the second sampler, the app exceeded the soft limit for currently opened files per process (=256). The crashes afterwards were caused by failing file handles in other parts of the app. A workaround can be found at https://stackoverflow.com/a/62074374/1993349 ... done.

I ran into the same issue. Did you ever find a solution?