Hello,
I recently experienced something similar, but to narrow the issue would you be able to include your Unity Log from before and during the ios build to Xcode process?
Additionally, if you are running in a simulator (or with an attached device) in XCode could you please separately include your Xcode log. Optionally, enable the flag -FIRAnalyticsVerboseLoggingEnabled in your projects Xcode scheme if prompted to in the logs. Instructions for how to do so can be found
here (also linked in the debug log itself).
Best,
Joe