Flat bar conversion - shifter is skipping a gear
Correcto, unfortunately for you, the latest (4700) Tiagra system uses a different cable pull to previous 10-speed groupsets, therefore the shifters are pulling the wrong amount of cable for your derailleur. It won't get better, however much you change the tension.

You could get a new 4700 derailleur, which would match the shifter and work with the existing cassette, keeping the flat bar.