I did a quick test changing the password using dietpi-config and I was not able to replicate the behaviour. For me, the password for user root as well as user dietpi was changed correctly
root@DietPi3:~# dietpi-config
[ SUB1 ] DietPi-Set_software > passwords ()
[ INFO ] DietPi-Set_software | "root" and "dietpi" login passwords successfully changed
[ OK ] passwords | Completed
root@DietPi3:~#