Applying without NOC followed by got selected and served more than two years. Now asking explanation for non disclosing

I have been an assistant professor at a central institute for over two years(25 months). I did not disclose that I am a state govt employee under probation period during application of assistant professor in Central govt. I disclose every thing during joining time and submitted all like NOC, released order, vigilance clearance. Institute allowed me to join as an assistant professor due to the availability of documents. I got an appointment letter approved by the executive council. I completed probation period one month before. Now I got a letter regarding non-disclosure of previous employment during application. The institute is now asking explanation within one week. As this is the time for issuing job confirmation order. I would like mention that central/State govt never issue NOC during probation period. Although I suppressed it due to probation period during the application for career growth but disclosed during joining time and submitted all documents. That time institute did not neither ask explanation or cancelled my joining. Now please help me with what to write in the explanation. Also institute has extended probation period that violates the rule that maximum probation period is 24 months.. I need help. please please help me.