Log shipping secondaries in STANDBY stuck in 'Restoring...' - SQL Server 2012 SP3
Please do restore verifyonly of your log backups just to make sure they are restorable. Your issue is similar to the one defined in  Log Shipping - RESTORE WITH STANDBY - on SQL Server 2012 keeps breaking. Try those suggestions as well.  

Few more things I would like to suggest is, if your log backups are valid try doing restores with NORECOVERY option to make sure the issue is with only STANDBY option. Also, I am assuming this is not an corruption issue. If it is then it is better to rebuild logshipping rather than fixing the corruption. This link should also help you out for some more suggestions: https://social.technet.microsoft.com/Forums/windowsserver/en-US/9c39fc61-0a59-402b-91fb-17ac1f03da7b/sql-2012-log-shipping-to-report-server?forum=sqldisasterrecovery