Sounds like you have something wrong with your layout design. Selecting or even entering a new value in a field on your main layout should have no automatic effect on the data in your related tables--no matter if "allow creation..." is enabled or not. Thus, I suspect something is not right with your design. (or there are some key details missing from what you have currently in place in your database.)
I would use a "new" button. If the user pulls down the value list and can't find the desired value, they click "new". A window, custom dialog or popover opens where they enter the data for a new entry in the value list and a script then takes the data from these fields and creates a new record in the related table.
Thanks Phil. That's my workaround for now, having the "new" button.
I created a very basic database to test out what I'm trying to do, but I don't know how to upload that here.
You can't upload files here, but you can upload the file to a file sharing site such as Drop Box and then post the down load link here.
Two other observations:
You can set a "unique values" validation rule in the values table to catch and prevent the generation of duplicate records.
With a bit of creativity, you can use a relationship with that "allow creation" option enabled such that a Set Field step from the context of the main table will create new records in the value table, but only if the value is not already present in the table. This would almost certainly require adding a second table occurrence for the values table using a different match field pair for the relationship.