Can I pray the Sunnah after the Fard Prayer without any valid reason? – Sheikh Assim Al Hakeem

There is a deobandi masjid where I happen to pray. Sometimes the Imam say ‘Allahu Aghbar’ instead of ‘Allahu Akbar’ in takbirs other than the opening one. But he doesn’t perform sujood as sahw at the end of prayer or even after Salam. Now do I have to repeat the prayers behind him (though he didn’t make mistake in pillars)?