Discarding Item Line does not discard all Tax lines

Hi

On certain invoices when we discard item line; its corresponding all Tax line (misc) does not discard automatically. Observed mostly its non recoverable CST line.

Is there any genuine scenario where its not allowed ? If we try to discard that tax line manually it gives message that "Discarding this line is not allowed as quantity billed/amount billed on the purchase order distribution would reduced below zero.

After upgrade from 11i to 12.2.5

thanks

Makar