Why does the salt value in /etc/shadow say "rounds=..."?

I have an embedded Linux device with a fairly modern Linux kernel. However, a strange thing happened with the last update: when a password is modified with passwd for any user, the salt value in /etc/shadow is replaced with rounds=65536, so an entry will look like root:$6$rounds=65536$6mA.... I suspect PAM is doing this. Does anyone have any idea what issue with PAM could cause such behaviour?