Is the marriage considered valid despite the explicit صيغة الإيجاب والقبول (formula of proposal and acceptance) not being mentioned at the time of the engagement and the contract?

The marriage is valid if its pillars and conditions are met, such as offer and acceptance, the guardian, and witnesses. If what occurred with the ma’dhun (authorized marriage official) included these, then the marriage is valid. Your husband's shyness preventing him from expressing his desire to marry you during the proposal does not invalidate it. If it did not include offer and acceptance, some scholars, such as Ibn Taymiyyah, hold that a marriage is valid if it is considered a marriage by common custom. This is a strong opinion that can be adopted to ward off waswas (whisperings/obsessive thoughts).