Yesterday "Atrium Harbour" was at rank 10.
I decided to test out upkeep mechanic.
My next payment for upgrading to rank 11 would have been 750 Gold, 24 Proteines and 48 Cereals.
I decided just to put in Upkeep for the current rank 10, means 600 Gold, 20 Proteine and 40 Cereals.

But instead that the rank stays stable at 10 as i was expecting because i hitted the rank 10 demands, it droped to rank 9.
Some days ago, when we missed to fill entirely, it feeled right, that we droped one rank down, because we did not even payed current rank demands.
It doesn't sound right, that you drop too, if you payed the rank. But maybe it is intended like this.
True, it can certainly be abused, if it works like i was expecting, but i would appreciate, if there would be some kind of period of grace, where you are able to hold/freeze your rank with just paying the current rank upkeep.
I will test today, what happens, if i just pay the rank 10 demands and not the debt.
Means 600 Gold, 40 Cereals and 20 Proteine.

IF we will stay tomorrow still at Rank 9, because i didn't payed the debt, but the upkeep, then you have almost the same effect, as if you just would stay at the current rank when you pay it.
Then it doesn't make sense to drop down as we did today. Except that you will be able to stay at a rank for the costs of the next rank.
I will update this tomorrow! Lets see, how it will be.
Update one day later:
Okay, we droped from Rank 9 to Rank 8, even i payed the Upkeep amount for Level 10 (but not the debt).
That system is weird and not to understand at all.
Well, i don't think it should be like this.
Also the description is wrong again:
