Description of problem: The unit has a typo:

User=rngd
Grou=rngd
 
This causes systemd to complain:

systemd[1]: /usr/lib/systemd/system/rngd.service:7: Unknown lvalue 'Grou' in section 'Service'

It does not actually break the privilege limiting, as systemd defaults Group= to the primary group of User=, which /proc/<rngd pid>/status confirms:

Uid:	995 	995	995	995
Gid:	992	992	992	992

But it's a wart nevertheless.

Version-Release number of selected component (if applicable):

rng-tools-6.6-3.el8.x86_64