The following attempt was done one 12.0-RELEASE.
# pkg install rabbitmq
Updating FreeBSD repository catalogue...
FreeBSD repository is up to date.
All repositories are up to date.
Checking integrity... done (0 conflicting)
The following 1 package(s) will be affected (of 0 checked):
New packages to be INSTALLED:
Number of packages to be installed: 1
The process will require 13 MiB more space.
Proceed with this action? [y/N]: y
[1/1] Installing rabbitmq-3.8.1_1...
===> Creating groups.
Using existing group 'rabbitmq'.
===> Creating users
Creating user 'rabbitmq' with uid '135'.
pw: user 'rabbitmq' disappeared during update
install: unknown user rabbitmq
pkg: PRE-INSTALL script failed
After rebuilding master.passwd (pwd_mkdb -p /etc/master.passwd) - the error ceased.
Closing as this is not a bug!