How to debug a growing Redo Queue Size in the Availability Group Dashboard?
There is no way to "debug" this other than to open an incident with Microsoft for investigation.  They may be able to get a series of process dumps or enable some more verbose logging to see if there is a hung thread, but you may have to leave it in the current condition for quite a while, so that is the downside.

Based on my experiences with the same symptoms, stopping and starting the SQL Server service on the secondary will resolve the issue (unless you have an underlying I/O issue that is causing it).  It may take a while for the database to recover as it will have to play all of the transactions in the redo queue, and during this time, replication will be paused.  You can track progress of database recovery in the error log, and once the database is recover, replication will resume.