Thanks, I emailed ehx support, and Chip confirmed the pedal should not act that way.

When I noticed the problem, I had the pedal on my pedalboard with other effects in front of in, with the EHX Deluxe Electric Mistress immediately in front. All effects were turned off except the Mel 9. Tonight I played through just the Mel 9 with no other effects in front, and there was no volume problem. So the Mel 9 is fine. Now I have to figure out which one in front of it was causing the problem.

I think I figured it out. I had my keyboard volume almost all the way up. I turned the keyboard volume all the way down to its minimum, and now I hear mostly the effect. Only if I hit the piano keys hard to I hear the dry sound.

That makes sense, since a line level coming out of a keyboard is a much higher level than a guitar signal.