Can't write to Sn_DHAR-Register

I try to send Data with the W5500-Chip, but according to use SEND_MAC-Command I have to write the Receiver(PC)-MAC to the Sn_DHAR-Register, but this doesn’t work. I always read out the default-value of 0xFF…FF. But: If the PC sends some Date to the W5500, the Sn_DHAR-Register is filled with the MAC of the PC. I wonder, if my cip is broken, or if I have to write a special sequence to another Register so I can write to the Sn_DHAR-Register.
(According to Datasheet Vers. 1.0 from August 2013 Sn_DHAR is read/writeable)

Hello, JoergH

To quote W5500 datasheet, SEND_MAC used with only UDP mode.

If you use W5500 with TCP, it is right to get PC MAC address.

Could you check if it is UDP or TCP ?

Thank you,