To reproduce, k-t-d:
1) Add a new subscription additional field, visit:
http://localhost:8081/cgi-bin/koha/admin/additional-fields.pl?tablename=subscription
2) Add a name, set MARC field '942$c' and MARC field mode = 'Get value from MARC record'. Hit 'save'.
3) Add a new subscription serial, visit:
http://localhost:8081/cgi-bin/koha/serials/subscription-add.pl
4) Input a biblionumber with data in it, e.g. '112'. Click 'Next' and 'Ok' on the alert box.
5) Notice the MARC field does not contain any data.
6) Fill in all mandatory fields. Hit 'Test prediction'. Hit 'save'.
7) Notice how the subscription screen does not show any MARC field.
8) Notice how nothing is saved in the database:
select * from additional_field_values;

I think step 5) should display the data since I believe that is the intended behavior. Anyone able to confirm this?