As per relevant provisions of section 73 & section 74, Where it appears to the PO that any tax has not been paid or short paid or erroneously refunded, or where input tax credit has been wrongly availed or utilised for any reason, he shall serve notice on the person chargeable with tax which has not been so paid or which has been so short paid or to whom the refund has erroneously been made, or who has wrongly availed or utilised input tax credit, requiring him to show cause (SCN) as to why he should not pay the amount specified in the notice along with interest & penalties.
In other words, to issue an order PO need to serve notice (u/s 73(1) or 74(1)) or statement (u/s 73(3) or 74(3))
Therefore, in the given case issue of additional order u/s 74 without any SCN or hearing is against the provision of CGST act. Hence, that additional order is not valid.
Also, technically what was never part of Show cause notice, can never be part of the order, Multiple High courts can be relied where in it was held.