1. In one independent thread, context, uses a AsyncHelper.RunSync(...) wrapper invoke mongodb's UpdateOneAsync method.
What "AsyncHelper" does is it replaces the SynchronizationContext.Current with a "ExclusiveSynchronizationContext", and replace back, one finished execution.

2. In another completely independent thread/context, executing following code:
code line 1...
... // SynchronizationContext.Current is null
await mongodb.UpdateOneAsync(); //with, or without .ConfigureAwait(false)
...// SynchronizationContext.Current is "ExclusiveSynchronizationContext"

These 2 calls not related to each other.

This is not 100% time happening. But when happen, it causes deadlock, or potential security issue as it leaks SynchronizationContext.

The code of AsyncHelper is attached.