I have a ML2 I bought last year (in a LaRue mount), and I took it out today to confirm it's zero. My shots were stringing vertically at 50 yds a good 4" up and down from my POA. I checked the bolts on the mount for tightness, and tried to see if I got any movement by trying to move the sight while it was mounted up. No dice. Then, I took off the sight, and tried my BUIS, and got a decent group right at POA.

This sight was used last year at Gunsite, and every time I've gone shooting since, and normally holds it's groups.

This is the first time, and it did it through 3 mags of 30 rounds.

Is this a "normal" mode of failure with the ML2 (not holding zero in one or both plane), or am I unique?

Vertical stringing is also a sign of your breathing during/through the trigger pull.  Watch your breathing or have another shooter re-confirm the problem you mention.