You create a Power Apps canvas app, add a form, bind it with the Common Data Service entity. On the form you add all the necessary fields. Now you want to save the form/record. You use the SubmitForm function in the OnSelect property of a Button or Icon to save or update the form but some of the fields value is not saved while the record is created or updated.
SubmitForm validates some of the behavior while submitting the record. Sometimes the Update property on the Card for the field is blank. If that happens, the field value is not saved or updated in CDS. Make sure the Update property on the fields Card is set while developing the Canvas App.