@edford: I appreciate your contribution to this thread. I encountered a similar situation. When I downloaded the crash reports from TestFlight, there was no 'reason' field. Then I read all the replies and your comments and found that I needed to check the crash reports using the XCode crashers organizer.
I am highlighting this for future readers:
To find the 'reason' field, It is important that you analyze the crash using XCode crashes organizer. Find your crash in the organizer, right-click and then click 'show in finder. This will be an XCcrashPoint file. Open it and the reason field should be there. See attached screenshot
Hope it helps.
Awais