This is simply the design of the HDBDD/CDS development object that it converts field names to upper case.
Here was the reason given by development for this design decision:
Reasoning for this:
If you write a “native” (meaning non-CDS-QL) SQL Select for the table, you don’t need to quote the simple names (those that contain no period), because they are generated upper case. Therefore we wanted the same for CDS.