We have the exact same issue.
Interestingly, due to hardware failure, our AG currently consists of only one (1) Replica, so there cannot be any issue with synchronization.
But we also had this issue before, when everything was normal.
The query from above states that log_send_queue_size and redo_queue_size are (as is expected) NULL.
The only solution seems to be to remove the database from the AG.