I have noticed that in the version 4.0.11 the service file for systemd changed and introduced a cyclic dependcy.
As it seems the "After" target has been changed in this version. Previously it was

After=network.target

and now it is

After=multi-user.target

This is an issue because the multi-user target is also set as the wanted by target:

WantedBy=multi-user.target

This creates cyclic depencies and from what I have noticed the order in which systemd fixes this cyclic depency seems to be somewhat random. This prevents all services depending on mongod.service to start at all and also seems to be randomly preventing other services from starting.

Once the system is up, all services can be started manully by hand, so it "only" affects the startup order. But since this definately breaks systems I've given it the priority "Critical - P2"