This can be reproduced by creating a form with data from a table that is linked
to another table by a Mastertable : Main table relation . For instance the field
mastertable.key_id links to the master table keys with fields
mastertable.key_id, mastertable.key_description. 
In the form.datasource there is only the key_id foreign key. 
By putting a dropdown list on the form i expect to link to the key_id field
while showing the values of the key_description fields. On save the underlaying
key_id of the selected value in the dropdown would be written to the form
datasource table key_id. 
The defect is you cannot. You can show the values of key_description allright,
but  it try to store this key_description value to the form.datasource.key_id is
stead of the key_id from the combobox. 

What did I do: 
- I placed a combobox on the form. The wizard appears.
- I select the master table keys.keys_description as the display field
- I select in the next step "Yes, I want to save it in the following database
field" the form.datasource.key_id (This is where you'd expect to link, or be
able to link, the mastertable.key_id to this field. now it links the
mastertable.key_description field to it. )
- Now the field shows the form.datasource.key_id (Should have been the
description from the mastertable) with a dropdown of all
mastertable.descriptions (Ok) 
- I now select a value from the dropdown.
-When saving the app complains I save the description into the database.

I tried to set the values manualy but came to the same concusion.