I know there aren't exactly linear curves in houdini only “straight” Curves which are beziers with aligned handles. This is fine as long as you don't move keys. In that case the aligned handles switch back to smooth which is extremely annoying. Is this a bug or “normal” behaviour?
If this is normal then SESI please, please, please add real linear curves, that stay linear until you tell them otherwise.

Edit: Forget the linear thing I just found the Qlinear mode. Still the slopes on straight curves should revert to zero when moving.

I think you want “linear()” as the function. I'm not sure what you mean by “slopes on straight curves should revert to zero”. The linear()/qlinear() functions do not use slope value at all.