Why is interest levied u/s 234a when return was filed before Dec 31st ?

I filed my returns on Dec 22nd 2020 and was surprised to see an interest levied u/s 234A amounting in excess of Rs. 8000 . The Auditor explained that it is a system generated amount and I was being penalised for not paying the pending tax by September 30th. I was not entirely convinced by this but I went ahead and paid the pending tax along with this interest.

I have not seen any notification where any pending tax had to be paid by 30th September. I am strongly of the opinion that both payment of pending tax (by self assessment) and filing of returns are done together. Is it possible that the IT Dept will realise this when scrutinising my filing and return this unfair penalty at a later date ? It seems that their eFiling software has not been updated to show that Dec 31st is the last date.

Thanks in advance for comments that may follow.