Citadels and tile buying

[Abraxis]

So I just ran into a curious situation, I'm not sure if it's a bug or just an unusual circumstance. I put down a citadel 2 tiles away from Mecca where Arabia had yet to claim. A few dozen turns later decided just to capture it. I annexed it a while after that and am just now looking to buy a tile to put down a customs house. The weird thing is, there are 4 tiles on the other side of the territory the citadel had originally claimed within the area of the city which can't be purchased... it's hard to explain, so here's a picture.

the plains tile two tiles to the left of the terrain tooltip is the one I want to buy, but can't.

Spoiler :

Now I don't know what to do, I don't want to waste a floodplain, put it on a foodless desert, or get rid of the iron mine, that plain tile is the best spot for it, but how do i get it, will I have to remove the citadel?

 

[wigwam]

Why would putting it on a floodplain be a waste, but putting it on a riverside plains tile not be?

What terrain is the Citadel on?

How many turns does a Marathon (I assume that's your speed) game usually go? Is it actually worth it to plant a Great Merchant instead of doing a trade mission?

 

[funkymunky]

I would guess your problem is that the city's culture hasn't actually expanded its borders to those tiles adjacent to the one you want to buy, despite the fact that your citadel's culture radius does include those tiles. You probably need to purchase the 240 gold tile highlighted in pink first, and then the mountain tile to the southeast of that before you can purchase the plains tile you actually want. Otherwise you may have to wait for the city's culture to organically expand the borders to those tiles before being able to purchase your plains tile. The game probably isn't factoring in the citadel acquired tiles when calculating which tiles your city can purchase; and while you and I might consider this a bug, it is probably just a game mechanic that didn't account for this particular situation.