When writing/reading to/from the OTP, the DW1000 system clock is divided from the crystal oscillator and not the PLL. This clock is much slower, and consequently, it will take more time to proceed the instruction. As a consequence the SPI clock needs to be reduced to 2Mhz. This is done in the simple example on git, for the DW1000 configuration.
You will successfully read the OTP by setting the SPI clock to the slow speed. Otherwise the value reported by the opt read command will not be valid.