Update:

it seemed identical.

Something has to be different for the test to fail. If the strings look identical then there must be some non-printing characters that differ: \r, \n, , , missing \0 ), etc.

Try printing your buffers byte-by-byte in hex for a few more bytes than the string length, to be sure there is an where it should be.

Or print the buffers with this hexdump function which lists a specified memory range in both ASCII and hex:

/***

Name:       hexdump

Function:   Dump memory contents to terminal

Description:Dump a specified range of memory in hex and ASCII

Parameters: byte *mem       - starting address
            uint16_t len    - # of bytes

Returns:    void

Notes:      Based on: http://grapsus.net/blog/post/Hexadecimal-dump-in-C
            Formats with sprintf(); outputs with Serial.print().
            Modified to print ...0 - ...F of each hex-block containing
            any of the requested memory range. Data not in the range are
            not shown.

***/



void hexdump(byte *mem, uint16_t len)
{
   byte *p, *pfirst, *plast;    // -> curr byte, first & last hex-blocks to print from
   char buf[10+1];              // sprintf o/p buffer


   // Print entire hex-blocks that contain the requested mem range,
   // except only show data within the range.
   pfirst = (byte *)((uint16_t)mem & 0xFFF0);           // beg of 1st  hex-block
   plast = (byte *)((uint16_t)(mem + len) | 0xF);       // end of last hex-block

   for( p = pfirst;  p <= plast;  ++p ){

      /* Print block addr */
      if( ((byte *)((uint16_t)p & 0xF)) == 0 ){
         sprintf_P(buf, PSTR("%06X: "), (unsigned int)p);
         Serial.print(buf);
      }

      // Print hex data, or if outside mem range, print spaces
      if( mem <= p && p < (mem + len) ){
         sprintf_P(buf, PSTR("%02hhX "), *p);
         Serial.print(buf);
      }
      else
         Serial.print("   ");                   // outside requested range - just spaces

      // Maybe print gutters and/or ASCII data
      if( ((uint16_t)p & 0xF) == 0x7 )
         Serial.print(" ");                     // narrow gutter after 8 bytes
      // If at end of hex block, print a gutter & re-print the block as ASCII
      if( ((uint16_t)p & 0xF) == 0xF ){
         Serial.print("  ");                    // wide gutter after 16 bytes

         // Print as ASCII.
         // Note: In this loop, we re-use the outer loop's index, 'p', to rescan
         // the hex block. We must leave 'p' as we found it!
         for( p = (byte *)((uint16_t)p & 0xFFF0);  ; ++p ){
            if( !(mem <= p && p < mem+len) )
               Serial.print(' ');               // not in requested mem range
            else if( !isprint(*p) )
               Serial.print('.');               // not printable
            else
               Serial.print(*(char *)p);        // print as ASCII

            if( ((uint16_t)p & 0xF) == 0xF ){
               break;                           // end of hex-block
            }
         }
         Serial.print('\n');
      }
   }
}