Hi Michael - it's not the reference plane of the original that is remembered in this case, it is the point that you picked to define the extrusion height.

When the extrusion is recalculated, it is using that same point to calculate a new height from the plane of the curve, that is why the height changes if you move it up and down.

There is workaround for this, which is to enter a specific distance for the extrusion height, instead of clicking a point to define the height. You can move your mouse around to get a good idea of what height you would like to use, but to finish the extrusion type in that height in the "Distance:" box in the upper-right corner, rather than clicking a point to finish the extrude.

That sets up the extrude history with a specific distance value instead of a point value, and it will then update later on using that same specific distance instead of calculating a new distance from the point.

I've been thinking of changing this to always save the distance value to history instead of keeping the point, because of that kind of unintuitive side effect that you noticed here.

- Michael