This has happened to me a number of times. Just now to upgrade one of my residential buildings, it said i needed 15 wood. So I went and bought 15 wood, but when I came back the upgrade for the building that needed the 15 wood was gone. A couple of the times this happened, I would upgrade different buildings and the upgrade for the building that disappeared would show up with the same things needed to upgrade it.
Yeah, this happens all the time to me as well. I believe that what is happening is that the game has decided that your upgrade will be to a different class of buildings (more luxurious, usually) and has changed the upgrade requirements to reflect this.
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