When submitting iOS apps to the App Store with Bitcode enabled, your app gets recompiled by Apple, to be optimized for specific devices and architectures. While Bitcode is optional, it's more and more encouraged by Apple, and even required for watchOS and tvOS apps.
Due to the fact that recompiling happens on Apple's servers, third party crash reporters don't have access to the symbolication files, which are required to properly associate the crash stack trace with the exact files and line numbers in your source code.
The current solution to solve this for Bitcode enabled apps is to manually download the dSYM files from Apple and upload them to your crash reporting service. Downloading the files can be done either using Xcode or iTunes Connect.