Marie,
If you have changed price_subtotal to float, then it must be added as a column in sale_order_line table at database(unlike functional fields, which are not stored in database so you can't find its column also). Odoo don't write the functional fields values to DB... as its functionally computed.
That might be one of the reason why the value price_subtotal is not writing to the DB.
But since you have changed it to float field, it must write its value to DB.
So, check your database and if its not there in the table(sale_order_line) then try updating your module again to reflect it to database.
Hope it helps!
it will be better if you include the code (changes you made)