Hi again, Pawel! Have you tried using an Aggregate step in the initial dataflow after you've added the column. You can choose whether the column is a measure or an Attribute. Please will you try that and then let me know how it goes, and i'll look into other things if it doesn't work? You can also try adding a CAST function into the "Add Column" formula to set the datatype - e.g. CAST(*column name* As numeric) and that will set the data type for that column!
In the end - I have managed it differently.
Basically, in my Dataset 2 I have set the 'Counter' column as text. Then during dataflow adding a new column works as expected. In the resulting dataset I'm changing the 'Counter' column back into number.
Not super elegant but it works for me.
Hope it helps,