Our 3rd party monitoring solution collects blocking information, but not for system threads. There was no additional information available for this blocking incident, but I could see that the system thread was a background process with the command “UNKNOWN TOKEN” and was sitting in a wait type of “HADR_WORK_QUEUE”. It was clearly the worker thread for the AG of a specific database.
A little later, we had blocking again involving that same thread, but this time, the AG worker thread was blocking the log backup thread. Seemed logical that if the worker thread could block the log backup, then the log backup could have also blocked the worker thread, but still it did not make sense to me.
This is one of those cases in which the answer makes perfect sense after the fact, but can be maddening until then.