Stefano,when you remove the alpha conversion from the infoobject then it might create some inconsistencies while loading data and also in reading the value.
Lets say firstly you loaded the data to object with Alpha conversion so value got stored like this :
3455 -> 0000000000003455
Now when you remove the Alpah conversion then value get stored as
3455 -> 3455
System will treat the above entries as two separate records and not single record.Check in system as well.
To display the data correctly you need to enter the value as they are like one with leading zeros and one without then you may be able to see both the records.
Hope this gives an idea.
Regards,
AL