I can not change sub-register 0x28:0C MSB byte value

The default value of sub-register 0x28:0C 0x de 1e 3d e0, as I use channel 5 so I wrote 0x 00 1e 3f e0 to dw1000 but when I read this sub register, it appears 0x de 1e 3f e0. No matter how many times I wrote to the MSB byte, 0x de will not change. I do not know if this would be problem as the MSB byte is reserved. Is anyone know what happend to me?