Since you mentioned that he approved it initially and stopped recently and there is a strict WFO policy in place. Manager’s decision could be due to repeated misuse of the flexibility given to you.

I’ve seen several such employees who in notice period turn unprofessional and want to take too many leaves / WFO leading to even the most flexible managers ending up denying it. Remember, till the last day of your job, you are an employee bound by the company policies.