@dickymac, this is expected behavior. When the cursor is in the melody area of Hookpad, playing notes on a MIDI controller adds melody notes. Hookpad will automatically figure out what scale degree to add to your song based on the key you are currently in. If you are in the key of C major and play a C, it will enter scale degree 1 (Do); play a D, and it will enter scale degree 2 (Re); play an E, and it will enter scale degree 3 (Mi)
When the cursor is in the chords area of Hookpad, playing notes on the MIDI controller adds the chord whose bass note is the note you play on the piano. So if you are in the key of C major and play a D on the MIDI controller, a ii (dm) chord will be inserted. Likewise, if you play G on the MIDI controller, a V (G) chord will be inserted.