Confirmation set as last delivery and unable to revoke it

Hello Experts,

There is an issue.We are using Extended classic scenario.

SC created for 100 quantities, PO for 100 quantities,

First GR --> 10 quantities  First IR --> 10 Quntities

Second IR --> 90 quntities, GR --> 60 and delivery set as completed.

Now they want to process GR for 30 quntities. But since delivery completed check is updated system is not allowing us to do one more GR. and also it is not allowing us to canccel the GR where user has updated "Delivery completed".

But in ECC by cancelling the GR document we can remove delivery completion indicator but thatr functionality is not there in SRM.

Is this behaviour of SRM system normal? or have we done any mistake. Kindly help me.

Thanks in advance.