Re-running the code in the latest release, R2024a, it appears the problem has been fixed.
Q=diag([0,1,1]./3^2);
axis([-5 5 -5 5 -5 5])
h1=fimplicit3(@(x,y,z) quadform(x,y,z,Q) ,'EdgeColor','none','FaceAlpha',0.3);
view([-2,11])
xlabel X, ylabel Y, zlabel Z
T=makehgtform('axisrotate',[0 1 0],pi/4);
h1.Parent=hgtransform('Matrix',T);
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1696336/image.png)
figure;
axis([-5 5 -5 5 -5 5])
h2=fimplicit3(@(x,y,z) quadform(x,y,z,Q) ,'EdgeColor','none','FaceAlpha',0.3);
view([-2,11])
xlabel X, ylabel Y, zlabel Z
T=makehgtform('axisrotate',[0 1 0],pi/4);
h2.Parent=hgtransform('Matrix',T);
axis([-5 5 -5 5 -5 5]*2) %<----makes the cylinder disappear!
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1696341/image.png)
function d=quadform(x,y,z, Q)
xyz=[x(:).'; y(:).'; z(:).'];
d=reshape( (sum((Q*xyz).*xyz)-1), size(x));
end