In the Patch function, the defaults() is used to create a new record. If you are editing a record you should use LookUp() or some other method to find the record being updated before you patch it.
quoting from the formula reference for Patch():
To modify a record, the base record needs to have come from a data source. The base record may have come through a gallery'sItemsproperty, been placed in acontext variable, or come through some other path. But you should be able to trace the base record back to the data source. This is important as the record will include additional information to help find the record again for modification.
So for your collection, the record you are editing should have an ID field derived from DB_A.that can be traced back: