Cannot lock to custom z-axis when extrude-scaling with custom orientation

Question

I have a part of a mesh selected, using a custom orientation called "ventil".

Now, if I want to add some thickness to the cylinder, by extrusion scaling it with

ESShiftZZ

I will be able to lock out the Z axis of the normal orientation in the extrusion, but not the custom orientation "ventil", which I actually expected would be the orientation to use.

Why?

Extrusions seems to only use "Normal", but grabbing uses the custom orientation.

Is this a bug?

(I would create the tag "Orientations" to this question if I had 300 reputation)