In the received RAW codes, the width of the modulated pulses is less than the expected value. And the width of the spaces is bigger than expected.

Use infrared receiver another model. For example TSOP4838.

Or try reducing MARK_EXCESS in the [IRremoteInt.h][1] of the Arduino-IRremote library from 100 to 50 or 0.

<!-- language-all: lang-cpp -->

    ...
    // Pulse parms are ((X*50)-100) for the Mark and ((X*50)+100) for the Space.
    // First MARK is the one after the long gap
    // Pulse parameters in uSec
    //
    
    // Due to sensor lag, when received, Marks  tend to be 100us too long and
    //                                   Spaces tend to be 100us too short
    #define MARK_EXCESS    0
    ...

And please increase the delay time in transmitter sketch (to avoid codes overlapping) :

    ...
    for (int i = 0; i < 3; i++){ 
      irsend.sendNEC(0xFF6897, 32); 
      delay(200); 
      j = j + 1; 
    }
    ...


  [1]: https://github.com/z3t0/Arduino-IRremote/blob/master/IRremoteInt.h