Well, you’ve got me stumped.  I have another thought, in the spirit of diagnostics by remote control.  It won’t solve the issue, but will help to narrow it down.

I assume that you have the EV-5 plugged into the EXP/CTL jack #1 in the back of the H90.  Switch it over to the #2 jack.  Go ahead & re-calibrate in System settings for EXP2.

Now try mapping the Program delay time to expression, and see if it also has the volume adjustment issue you were hearing.  Just remember that you have to ‘drop down’ to assign to EXP2, or use the LEARN mapping function.