Unfortunately, this appears to be working as designed. In the future, I would recommend not building the Mercenary Camps. Also avoid building the Order of Asok, as Knights of Asok take 9 gold per turn each, and there are three units of them, but they are nothing like worth it (though they are closer to being worth it than the mercenaries and hunters).

This has come up several times in the past, and as far as I know none of the developers have commented on it.

The most recent discussion of this that I know of is here:

https://forums.elementalgame.com/436191

Hmm last few games i built it i had no maintenance cost. Although that was before the last update or two. They are supposed to be zero maintenance units.