If you have low KH and add bakign soda, your pH should go up all other things being equal, if the KH is 3 and you raise it to 6 etc.
Sounds like a lousy pH test kit or a bad KH test kit.

Test the KH out on a DI water smaple, slowly add more baking soda and the pH should buffer to about 8.1-8.3 range or so as you add more.

Regards,
Tom Barr