Surprised?  The method for getting a control to store its value in a field
is to place the name of the field in the control's ControlSource property.
You have not done that.  You have an expression in there.  A control is
either bound to a field OR contains an expression.  It cannot do both.

The good news is that proper database design dictates that you should not be
storing that value in your table anyway.  Just remove that field from your
table and any place you need to see that value use the expression you are
using now in your form.

Tables should contain raw data only, not data that can be derived from other
data.  That is the job of queries, forms, and reports to calculate in real
time as needed.  That way there is never any doubt as to its correctness.