Your statement that the position is correct led me to research the stamp with my minimicroscope.
The 2 horizontal lines caught my attention.(not very good photo)
The stamp is printed in offset and I think that the flaw is not caused in the "plate cylinder" but by the existence of a small scratch in the "offset blanket" cylinder (rubber blanket).
A small relative axial displacement of the two cylinders can lead to total or partial cutting of the left M leg.
Notice the accumulation of ink up and down the cut.
Any opinion is helpful to me.