Why is there progress callback information printed when using backgroundSession to upload tasks in the background

Question

I would like to ask a question when I was in the background with backgroundSessionConfigurationWithIdentifier upload task encountered the following problem:

Background: I have a lot of uploading task (assuming that there are 200), I will create 200 jobs in a for loop, to join the queue, queue maximum number of concurrent is 4, I use backgroundSession to upload, and I implements handleEventsForBackgroundURLSession method in the APPDelegate, and print some info (interceptApplication called for URLSession), then implements NSURLSession URLSessionDidFinishEventsForBackgroundURLSession agent method, and invokes the handleEventsForBackgroundURLSession completionHandler (and print the log URLSessionDidFinishEventsForBackgroundURLSession).

Question:

1. I found that the progress information will be print after program enter background (but I checked the information and said that there would be no progress callback during background upload, and only called didCompleteWithError method for each task after all background tasks were completed).
2. Because the task a lot, I am in handleEventsForBackgroundURLSession and URLSessionDidFinishEventsForBackgroundURLSession output information will be printed many times, I thought that all task can be completed, but I find the program will stop printing log message after Running for a period of time.

Any help is appreciated.