node, 136 bytes

Result

AC

Duration

860ms

Code [DL]

a=require("fs").readFileSync(0);for(i=51;i<a.length;i++)if(i%51<50)a[i]|=a[i-51];i-=51;while(a[++i]!=116)a[i]|=a[i-1];console.log(""+a);

stdin

                              T                   
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
    K                                             

stdout

                              T                   
                              t                   
                              t                   
                              t                   
                              t                   
                              t                   
                              t                   
                              t                   
                              t                   
    kkkkkkkkkkkkkkkkkkkkkkkkkkt                   

stderr

strace