I encountered this problem in Form, which is I couldn’t send a data column to the spreadsheet (it made an empty cell) if the column is created by “Lookup” of a relation.
Then I found it works correctly if I use the column as a component.
As a workaround, because I did not make it appear, I made a template of <!--ID--> where ID is the column I wanted to send to the spreadsheet. Then I used the template as a component of the form.
Finally, boom!!
I am posting this because I thought a data column alone should be sent to the spreadsheet correctly in the first place.
I don’t seem to have a problem doing this, unless I’m misunderstanding what you are trying to do. I have a main sheet with a lookup column that gets a value from a different sheet. When I am viewing the details of an item from the main sheet, I create a form button and then I add a COLUMNS component that contains the value from the lookup. When I submit the form, that value is added to the sheet.