I agree with orange...I've read your post numerous times and can't really follow it (also probably why four dozen members have read it and only two of us have responded)...a clearer explanation is really needed...but here's a couple of things.
If the Subform is Bound to a Table...when you exit it, any Bound Fields will automatically be saved...with no action needed, on your part.
If this isn't happening, then either your Field isn't be Bound, or (and going by the line "does not create a new record in the data table") I suspect that you're not adding any other data to the Subform...hence it's not dirtied; adding data to a Record thru code only does not dirty the Form) and not being dirtied, no Record is there to be Saved!
I have to tell you, having a Subform that will only ever contain one Record, i.e. in a one-to-one relationship, is generally bad design! You don't need this is order to Query it out later, when the same data is available in the Table the Main Form is based on.
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007