withholding and amount split in MIRO

Dear Consultants,

we activates amount split in MIRO and after this as you might know you are able to split your amount in order to have same withholding tax type but with different tax code. As long as we choose a tax code everything is working well but as soon as we want to enter the value manually, system doesn't save the amount.

Are there any solutions to enter withholding tax manually in amount split in MIRO or FB60?

Any help would be appreciated.

Best Regards,