When installing tpm-emulator as a dependency for another packages with pkg:
[2/49] Installing tpm-emulator-0.7.4_2...
===> Creating groups.
Using existing group '_tss'.
===> Creating users
Creating user '_tss' with uid '601'.
pw: user '_tss' disappeared during update
pkg: PRE-INSTALL script failed
I'm not sure why this is occurring, but doing:
pwd_mkdb -p /etc/master.passwd
will fix the issue.