@vishwa804 Did the say anything more than to "flash update the phone manually"? Unclear if they mean to factory reset the device, however that would be an option. Especially seeing how your earbuds worked fine with another ROG Phone 3 at the service center.
Before you consider doing that, you can reset your bluetooth settings and see if that solves your issue:
Settings -> System -> Reset options -> Reset Wi-Fi, mobile & Bluetooth