node, 124 bytes

Result

AC

Duration

850ms

Code [DL]

a=require("fs").readFileSync(0);for(i=51;i<a.length;i++)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   
                                              t   
                                           kkkt   

stderr

strace