The Amount Segment gets greyed out when saving the Distribution

Hi,

I am having the following problem in the Invoice Workbench:

 

1. Enter Line Information
2. Select Tax Classification code
3. Click on Distribution and Save

-   The Amount segment (on the Item line) in Distribution and Line gets greyed out.

- The Invoice has the status Never Validated.

 

Note: If I enter a new Invoice with the same info. Except I do not select a Tax Classification Code on the Line level the Amount segment is updatable after I save.

Does anyone know if this is a bug or if it´s a setup issue?

I need the functionality to be in the way that I can change the amount on the Item Line until I validate the Invoice.