Well...no offense, but I have noticed that people never believe the claim that the effect is bugged. And I made the mistake to not use proof from the start.
So I decided to settle this once for all so that we can move on to fixing it. It's quite an important faction trait for such a major faction of the game. Well, here is proof.
So the test is done with a general with no other cost reduction effects applied (like cunning). We see the germanic warband has the same upkeep cost for both ERE and WRE, so the WRE faction trait doesn't apply at all in this case.
ERE upkeep: 153
WRE upkeep: 153
Let's recruit a few non-roman units and see if the upkeep will be reduced according to the trait's description
Nope. Conclusion is the effect doesn't work.
And now, a confirmation that IF it worked, we would have seen it: here is a general with 5 cunning which provides upkeep cost reduction
We see with this that with an effect that does work, we do see the number going down (146 instead of 153). I also want to note here that roman units didn't get an upkeep reduction either.
I hope I have proven my claims now, sorry for the previous wall of text
So, is there anyone interested in helping me fix this?