Hold on I have to go edit the rest of my responses because I just assumed you wrote the code correctly; you did not.
You are not hashing between calls to the timer. The sha256 hash itself is responsible for doing physical things to the chip (heating up some parts unevenly during the hashing computation) which introduces meaningful entropy between calls to the current time.
You can't just do calls to clock_gettime(), you have do an actual sequential sha256() call between them. Please run this code again and tell me what results you get.
You are not hashing between calls to the timer. The sha256 hash itself is responsible for doing physical things to the chip (heating up some parts unevenly during the hashing computation) which introduces meaningful entropy between calls to the current time.
You can't just do calls to clock_gettime(), you have do an actual sequential sha256() call between them. Please run this code again and tell me what results you get.