From the attached file I find the dismissal order was for:-
1. Leaving headquarters without permission.
2. Not reporting for duty despite calling upon you to report for duty.
3. tampering with a sick certificate.
Tampering is only incidental. The main reason is not reporting for duty despite calling upon you to do so. In other words disobedience.
Was the doctor examined in the inquiry to prove tampering? What prevented you from examining the concerned doctor to prove that there was no tampering? Probably, doctor must have been reluctant to appear as a witness in the inquiry at your instance.