Why the IF condition gets FALSE even though the complete DS is blank?

Here's a test source where I need to ID and validate the (fixed) *blank positions in a fixed-len string, data. So here the first blank_string is of char(4) starting pos(2), and the second blank_string is of char(2) starting pos(8) declared as a source DS

dcl-s data char(12) inz('z    zz  zzz');    
                                            
// '1234+6789112'                           
// 'z....zz..zzz'                           
                                            
dcl-ds source inz;                
  *n char(4) pos(2); //will be usually blank
  *n char(2) pos(8); //will be usually blank
end-ds;                                     
                                            
source = data;                              
                                            
//check if the DS is blank                  
if source = *blanks;                        
  dsply 'DS is surely blank';                      
endif;                                      

looking in debugger, the DS appears as expected, i.e. all blank.

Even though the DS is as clean as a glass, yet the IF condition is set to FALSE, and exits without executing the dsply.

EDIT: summarizing compiler's POV

//structure user trying to define

dcl-ds source inz;
  *n char(4) pos(2); //will be usually blank
  *n char(2) pos(8); //will be usually blank
end-ds;

//structure that actually got defined

dcl-ds source inz;                
  *n char(1) pos(1);
  *n char(4) pos(2); //will be usually blank
  *n char(2) pos(6);
  *n char(2) pos(8); //will be usually blank
end-ds;  

Because source isn't blank. source contains 'z zz ' as you copied data to source.

Just because the debugger by default is showing you the defined sub-fields of source doesn't make that all that is in source.

try this eval source:c 9

Which instructs the debugger to display source as a char(9) variable.