The LPC_SYSCON->DEVICID register is specified to be at offset 0x3F4 in the manual (UM10732), but it is actually located at offset 0x3F8. Reading LPC_SYSCON->DEVICID thus does not yield a valid part ID.
Workaround: The IAP function to read the part ID reads from the correct location, so just use this function.
Source Code change: Change line 111 in syscon_11u6x.h to read:
__I uint32_t RESERVED16;
This moves the following DEVICEID field up to the correct offset.
Please change the address in the manual.