@AndiW I cannot confirm this. Is there a chance you installed a different decoration? If I add
\begin{tikzpicture}
\begin{scope}[declare function={varyingamp(\x)=0;}]
\draw[help lines] (0.22,0) -- (2,0) .. controls (2.35,0) .. (2.5,-1.25) .. controls (3,-4.0) and (3.5,0.25) .. (5.75,0) -- (11,0);
\draw[postaction={decorate,decoration=mark varying random y steps}] (0.22,0) -- (2,0) .. controls (2.35,0) .. (2.5,-1.25) .. controls (3,-4.0) and (3.5,0.25) .. (5.75,0) -- (11,0);
%\path[decorate] (0.22,0) -- (1.9,0) .. controls (2.25,0) .. (2.4,-1.5) .. controls (2.9,-5.0) and (3.7,-1) .. (5.75,0…