What is the ruling on praying behind an Imam who does not completely wipe his hair during ablution, knowing that I prayed behind him once out of ignorance, and another time out of forgetfulness?

Wiping the head is one of the pillars of ablution, and scholars have differed regarding the amount that must be wiped. The Shafi'is hold that wiping three hairs is sufficient, and it was also said: one hair. The Hanafis hold that wiping a quarter of the head is sufficient. Your prayer behind someone who does not wipe their entire head is valid, because the matter is subject to juristic interpretation (ijtihad (definition)), and praying behind those who differ in secondary matters (furu') is permissible and not disliked by scholars. The Ummah has reached a consensus on this. We advise you to ignore obsessive whisperings (waswas) and not pay attention to them.