The mongod service unit is configured to be part of the multi-user.target, but the unit configuration also has the following parameter:
Basically, this makes mongod start just after the network is up and not during multi-user (one of the latest targets to trigger). This causes problems on those servers where there are network storage services or authentication services (like AD) still pending to start. mongod would not be able to start if it depends on them.
Removing the After parameter would make systemd to start mongod as part of the correct target. If an After is needed for some reason, After=multi-user.target would avoid most of the problems with dependencies between services.
Affects all native packages (RPM and DEB) for systemd based Linux distros.