log_reuse_wait_desc - AVAILABILITY_REPLICA Unable to truncate transaction log
The reason you are seeing this behaviour is due to the fact that you have SUSPENDED the data synchronization (see @AMtwo's comment).
From BOL,

a secondary replica must be able to communicate with the primary replica and must be SYNCHRONIZED or SYNCHRONIZING.

so if you unsuspend the data synchronization, then the log can be reused since you took the log backup.