I spent the day thinking about this issue and concluded that:
1 by the time I discovered the pitch-down moment, the wheels were blocked, as if they had brakes.
2 in such case, there will be a pitch-down moment caused by the thrust vector multiplied by the distance between ground and the thrust vector.

I hang the plane by its center of gravity and applied power. The elevator responded well, for pitch up and down, at various leves of power.
Let's go for another tentative flight and see what happens.