Let me guess, you're using TRS cable, right?
Please try TS cable, then most likely the volume pedal will behave upside down, meaning heel down is max volume and toe down is silence. Then invert the tip polarity from global setting to make it normal (heel down silence and tow down max volume).
Volume (or any expression parameter) max at the middle, and min at both end is an expected symptom if you pair TRS style pedal with HX using TRS cable.
HTH