iOS missing system symbols, even after uploading them

We're getting intermittent crashes on iOS on some devices, and no crashes at all on others. The crash always produces the stack trace below, which contains nothing useful. It shows that some worker thread is crashing, which explains why the crashes are intermittent, but nothing about what actual code is crashing.

The last frame says "system symbols missing", so I've uploaded the system symbols manually for iOS 11.3, and 11.3.1 from here:

After uploading those symbols and waiting about an hour, the crash report remains the same.

I have no idea how to debug this problem at this point, because we don't have physical access to any of the devices that are crashing.

How do I figure out where this crash is coming from?

Thread 16 (crashed)

0   .                          0xffffffff00000000 <system symbols missing>

1   our_app                    0x0000000105d09ca8 JobQueue::Exec(JobInfo*, long long, int)

2   our_app                    0x0000000105d09c14 JobQueue::Steal(JobGroup*, JobInfo*, long long, int, bool) 

3   our_app                    0x0000000105d09fec JobQueue::ExecuteJobFromQueue()

4   our_app                    0x0000000105d0a0cc JobQueue::ProcessJobs(void*)

5   our_app                    0x0000000105d096ec JobQueue::WorkLoop(void*)

6   our_app                    0x0000000105decd48 Thread::RunThreadWrapper(void*)

7   libsystem_pthread.dylib    0x0000000184081220 _pthread_body

8   libsystem_pthread.dylib    0x0000000184081110 _pthread_body

9   libsystem_pthread.dylib    0x000000018407fb10 thread_start