Combine multiple implicit surfaces in a stl file

Dear community,

I am trying to write and export several simultaneously created implicit surfaces to a common stl file.

If I export the two surfaces separately I get two stl files with correct surfaces, but as soon as I combine the data in Matlab the second surface is "destroyed".

Enclosed are the images of the separately created and the common stl document and my code for the common variant.

Many thanks already and best regards

clc
clear
close all


n=60;
t=pi/n;

x_max=1;
y_max=1;
z_max=1;
r_max=6;
r_vor=2*pi*r_max;
factor=pi./6;

xi = -x_max/2:t:x_max/2;
yi = -y_max/2:t:y_max/2;
zi = -z_max/2:t:z_max/2;

[x,y,z] = meshgrid(xi,yi,zi);

F=cos(2.*pi.*x).*sin(2.*pi.*y)+cos(2.*pi.*y).*sin(2.*pi.*z)+cos(2.*pi.*z).*sin(2.*pi.*x); %   G

% First body
[fs,v]=isosurface(x,y,z,F,0);
v(:,2)=v(:,2)+r_max;
[~,rho] = cart2pol(v(:,1),v(:,2));
theta=v(:,1).*factor;
[v(:,1),v(:,2)] = pol2cart(theta,rho);

[fc,v2,c] = isocaps(x,y,z,F,0);
v2(:,2)=v2(:,2)+r_max;
[~,rho2] = cart2pol(v2(:,1),v2(:,2));
theta2=v2(:,1).*factor;
[v2(:,1),v2(:,2)] = pol2cart(theta2,rho2);


fn = [fs ; fc+length(v(:,1))];
vn = [v ; v2];
 

% Second body
[fs99,v99]=isosurface(x,y,z,F,0);
v99(:,2)=v99(:,2)+r_max;
[~,rho99] = cart2pol(v99(:,1),v99(:,2));
theta99=v99(:,1).*factor;
[v99(:,1),v99(:,2)] = pol2cart(theta99,rho99);
phi=30/360*2*pi;
R99=[cos(phi) -sin(phi) 0;sin(phi) cos(phi) 0;0 0 1];
for i=1:length(v99(:,1))
    v99(i,:)=R99*v99(i,:)';
end


[fc99,v299,c99] = isocaps(x,y,z,F,0);
v299(:,2)=v299(:,2)+r_max;
[~,rho299] = cart2pol(v299(:,1),v299(:,2));
theta299=v299(:,1).*factor;
[v299(:,1),v299(:,2)] = pol2cart(theta299,rho299);
for i=1:length(v299(:,1))
    v299(i,:)=R99*v299(i,:)';
end

fn99 = [fs99 ; fc99+length(v99(:,1))];
fn99 = fn99+max(max(fn));
vn99 = [v99 ; v299];


% Combinition and Output using function stlwrite
fnges = [fn ; fn99];
vnges = [vn ; vn99];
stlwrite('geometry.stl',fnges,vnges);